The great thing about song writing is that there are no hard-fast rules. The best advice I can give is to write what you know and don't sensor yourself or edit yourself too much. It is not necessary for lyrics to rhyme. Nor do you have to stick to a particular format. With that said, most popular songs at least loosely follow this basic structure: Verse; chorus; verse; chorus; bridge; and usually either another verse or another chorus or back to the start. The pre-chorus which connects verses to the chorus is also found in a lot of cases.