The sonnet is one of the fixed forms in poetry. It is made up of fourteen lines of rhymed iambic pentameter. Iambic pentameter is a line made up of five beats. If you are having trouble with iambic pentameter read some Shakespeare. Or follow this line out of Shakespeare's Sonnet 18 "Shall I compare thee to a summer's day?"
There are various forms of sonnets and each have different rhyme patterns.
The original form of the sonnet is called the Italian sonnet or the Petrarchan sonnet. The rhyme pattern could be an octave abbaabba and a seset of cdcdcd, cdecde, or cddcee. Those are the possible patterns. The Italian sonnet is usually made up into two stanzas with a volta or turning point in the middle. A volta signals a change of thought in the poem. The volta may begin with "but" or "then."
An example of a Italian sonnet is:
Holy Sonnet 10 By John Donne
Death, be not proud, though some have called thee// Mighty and dreadful, for thou art not so;// For those whom thou think'st though dost overthrow// Die not, poor Death, nor yet canst thou kill me.// From rest and sleep, which but thy pictures be,// Much pleasure; then from thee much more must flow,// And soonest our best men with thee do go,// Rest of their bones, and soul's delivery.// Thou'art slave to fate, chance, kings, and desperate men,// And dost with poison, war, and sickness dwell,// And poppy or charms can make us sleep as well// And better than thy stroke; why swell'st thou then?// One short sleep past, we wake eternally,// And death shall be no more; Death, thou shalt die.
Another form of the sonnet is the English sonnet or the Shakespearean sonnet. (Same sonnet, two names.) The English sonnet has a rhyme scheme of ababcdcdefefgg. It is usually one stanza long. English poems can have voltas too.
Here is an example of an English sonnet:
Sonnet 18 By William Shakespeare
Shall I compare thee to a summer's day?// Thou art more lovely and more temperate:// Rough winds do shake the darling buds of May,// And summer's lease hath all too short a date:// Sometimes too hot the eye of heaven shines,// And often is his gold complexion dimmed;// And every fair form fair sometimes declines,// By chance or nature's changing course untrimmed;// But they eternal summer shall not fade,// Nor lose possession of that fair thou ow'st;// Nor shall death brag thou wander'st in his shade,// When in eternal lines to time thou grow'st:// So long as men can breathe, or eyes can see,// So long lives this, and this gives life to thee.
There are other sonnets with different rhyme schemes, and are usually called Nonce sonnets.