Do i need the books

Im new here and before i join a group i would like to know if I should buy the books? if any one could let me know i would really appreciate it.

That really depends on the Game Master for whichever group you're playing with.  Roll20 is a system to enable playing, but it doesn't have rules systems built into it. So, if you Game Master says you need to have a copy of the books, then yes, but if s/he doesn't, then you wouldn't need them.

It depends a lot on the GM and particularly the game. If you want to play say DnD 4E (why would you!) you would need a book to have a resource for all the things it has. On the opposite side, Pathfinder (3.5 has resources online as well) has several places for all its information such as the Pathfinder SRD which has everything you need to know online without needing the physical book.  It all depends what System the GM is using, what they are willing to do to help you start off (systems like DnD/pathfinder would be extremely difficult without having resources without the GM making your character essentially), and just how homebrewed the system might be.

Game resource thread  : take a look through this thread. There are a lot of links that might help or confuse you but if you get confuse, just ask away and someone will answer to help. Paizo pathfinder SRD Pathfinder OGC SRD WoC D&D 4E SRD WoC d20 SRD The Hypertext d20 SRD SRD News (many srds listed here) d20 Modern SRD Hope all the srd links help you out.

Hi, Joe P.!  Welcome to the Roll20 forums.  I think the friendly forum-goers above have explained things quite well.  GM's are almost universally receptive to questions from potential players, so don't hesitate to ask if there's anything you want to know before joining a game.  

Speaking as a GM, before you start investing in books, find a new-player-friendly game that interests you and ask if you can join. Many systems have free quick-start rules that you can try out to see if you like the system.