Selecting Seeds 
Good marijuana simply can not be harvested from poor seed. There are thousands of strains of marijuana and established seed lines and breeders. All of these have different characteristics for you to select from. Among the traits to consider are size, duration of flowering period, yield, type of psychoactive effect, strength of psychoactive effect, THC content (this is NOT synonymous with psychoactive potency), odor, taste, and visual effect.
Quality of Source 
The quality of source seeds is something you need to consider. Your sources can range from bag seed from dirt weed to clones from a plant with known premium pedigree properties.
Any grower will tell you that smokers don't know what is good and what is not. There are no shortage of smokers who have only experienced good marijuana in the form of a midgrade. These individuals tend to refer to any decent mid as skunk or chronic. There is nothing wrong with that but if you are one of those individuals you are about to attain a new level of awareness.
One thing you should note is that the quality of the buzz is actually more important than the potency. It really doesn't matter how potent the marijuana is or how high the THC content. A plant with a strong couchlock or cerebral high will have a peak effect and the potency of the plant only changes how much material must be used to achieve that effect. The end result of smoking lots of low potency marijuana is the same as smoking a smaller amount of the most potent and highest thc content marijuana with the same kind of buzz. This can be observed by smoking a sinsemillia bud and a pollinated bud from the same mother. Both will have the same effect, you just have to smoke more of the lower potency seedy material.
Bag Weed 
Bag weed is a mystery. The genetics hiding in those seeds could hold many surprises and could hold the next great pedigree plant or they could hold nothing more than dirt weed. When evaluating bag weed there are a number of indicators to look for. The first is packaging. If the marijuana is packed in a brick or pressed into tight chunks that is has to be broken off of then this indicates that it is a heavily commercialized strain. This is fairly consistently low quality material. The next indicator is seediness. It is perfectly possible to lower the quality of premium marijuana by pollinating it but this sort of carelessness indicates that the grower either didn't care about what he was doing or didn't know. It is a fair bet that said grower didn't care or know about the strain either. Another important indicator is color of the plant material. A very dark brown, almost black indicates a highly competent curing method, next up is a nice green plant material that doesn't smell grassy and reveals the color of the hairs, purple plant material will tend to have less potency potential among pedigrees but has excellent bag appeal and generally will be of a pedigree strain, the absolute worst things to see are a light green plant material with a grassy smell and light brown material.
Now having considered those qualities you will of course want to consider the buzz delivered by the material, side effects like paranoia, dry mouth, and appetite stimulation. Also important is odor and appearance. Not all pedigree marijuana has a strong odor but a strong odor is usually a good sign. And of course taste is important. Some marijuana strains even have a nice fruity taste.
Purchased Seeds 
One easy way to acquire excellent genetics is to simply purchase from a known and reputable breeder. There are many online seed companies that are reliable and will ship anywhere in the world. You do need to be cautious because the companies do not verify that seeds are legal in your location and leave that burden upon you. If you try to import seeds where they are not allowed by law you risk losing the money you invested in the seeds.
Things to look for in seeds are feminization (if done by a competent breeder these seeds do NOT carry an increased probability of hermaphroditism). Feminized seeds produce all female plants. This is desirable since in regular seed only about half are females. You will want an all female crop in order to prevent pollination and grow sinsemillia weed that is much more potent. Since only the female plants are desired half of the seeds you purchase will be wasted with regular seeds. It will also leave empty space in your grow area.
Another important trait to look for are seeds which are F1 Hybrids. F1 Hybrids are genetically more desirable. These seeds are from the strains original parent plants. Seeds from two F1 plants of the same strain would be F2. With each successive generation of inbreeding the genetic stock becomes weaker. F1 plants will also be more likely to enjoy what is called 'hybrid vigor'. Hybrid vigor is a phenomenon that sometimes occurs in hybrid crosses causing a incredible vigor and growth in the offspring produced by the cross.
Clones obviously aren't seeds, but they are cuttings from a mother plant and used to propagate genetics and start gardens. When grown out the cutting will have the same genetics as the mother plant and therefore all the same characteristics. There are locations where you can sample marijuana and then purchase cuttings from the same mother as the plant that produced it. Obtaining a cutting of known quality genetics is the best way to obtain genetics. If this option is available to you then you should go with it. Cuttings can be taken from any marijuana plant so if you have a friend that grows you might be able to acquire excellent genetics. As an added bonus, since cuttings are clones they also share the gender of the parent plant. This means cuttings from a female plant will always be female.
Basic Types of Plant 
It is worth having some understanding of what strains of plant are out there. This lets you describe what you are growing and understand what others are growing.
Cannabis Indica 
Indica plants are short, bushy, mature early, have more chlorophyll and less accessory pigments (accessory pigments protect the plant from excessive sunlight). Indicas are very resinous plants that provide a body buzz couch lock effect.
Cannabis Sativa 
Sativa plants are taller, take longer to mature, have less chlorophyll and more accessory pigments. Traditional botonists tend to mislabel all cannabis as cannabis sativa. Sativa plants have much higher yields than indicas and have a very cerebral buzz. They are typically very consistent maintaining the same results from one generation to the next.
Cannabis Ruderalis 
C. ruderalis plants are typically very short and extremely low potency. Ruderalis strains do have one interesting trait: other strains of marijuana flower when the light output is reduced to twelve hours a day or less, allowing a build up of a light sensitive hormone in the plant that tells it winter has arrived. C. ruderalis plants automatically flower after a certain time period independent from light output and are, therefore, referred to as autoflowering. For this reason there are crosses between potent indicas and sativas with ruderalis to attempt to create a plant with higher yield and greater potency that flowers automatically. This offers the benefit of having the plant enjoy as much photosynthesis during its life cycle as possible.
A hybrid is a cross between two or more of the other strains of plant. Most commercially available strains are hybrids with varied amounts of indica and sativa. There are a few ruderalis hybrids as well.
Germinating Seeds 
A great deal is made of germinating seeds. In truth the process is simple and if done correctly will give excellent results, generally 90+% germination rates.
In order to germinate seeds you should take a paper towel and fold it in half and half again. Wet the paper towel thoroughly with reverse osmosis filtered water and place the seeds in the crevice made by the last fold. Close the paper towel and place it on a plate. Then place the plate in a dark place and check it 24hrs later. Continue to check every 8hrs keeping the paper towel moist until the shell of the seed opens and a white shoot comes out. This will typically occur within 72hrs. Now that the seed has sprouted you can push it into your desired grow medium. It is best to sprout your seeds in either a small rockwool cube or directly into the same grow medium you plan to use in your final grow. This will reduce stress on the plant when you transplant it.
From Sprout to Seedling 
Once the germinated seed is placed in grow medium it should be watered with plain water until it sprouts. When it sprouts it will have two small round petals. This is considered a sprout. You should continue to water with clean water at least until the first set of true serrated edged leaves form and the two small round leave brown and die. This is normal, the plant uses those leaves as a sort of parachute to allow it to utilize the nutrients stored in the seed itself. During this stage of growth the plant is very delicate and should only be placed under fluorescent lights.
At this point your sprout is now called a seedling. If you sprouted in a rockwool cube you should either place it into your desired grow medium and begin feeding with a weak 1/4 strength nutrient solution or if you have it in soil you can just continue to water as is since there should be nutrients in the soil.