To understand numbers in Haitian Creole, it is helpful to know French (but not necessary) because they are all derived from that language. The Haitian Creole numbers are mostly phonetic representations of how the same numbers are pronounced in French (changed according to a few rules), so if you know some rules regarding French pronunciation, you will understand Haitian Creole numbers more readily.
Twenty is ven. For the rest, you take either vent- or venn- and add the units number. Use vent- for 1, 8 and 9. The only units number that is different from before is 1, which is eyen instead of en (which comes from French et-un, "and-one").
These are irregular. You take the word for "sixty" (swasant) and add the numbers 10–19 as though it were "sixty-ten" for "seventy". This is also derived from French. Note that where in French you would say soixante-et-onze (71), you say swasantonz, and not *swasanteyonz.
A worksheet covering this material is available at Wikiversity.