The sooner the better imo.
When I was pondering over making a deposit to play online poker (I'd never downloaded or played any poker online before depositing) I waited for a few weeks, did a small bit of research (mostly asking friends who I knew played poker online... didn't get much info. from them at all though) then signed up.
I only wished I'd started sooner! Seriously,... the first while playing poker online was a blast! I loved it! I'm sure you'll have a simliar experience. It might take a bit before you become a winning/break-even player but the small buyins you can play are well worth the investment for entertainment alone (not to mention for the experience they'll give you on your journey to becoming a better player).
I say 'Go for it!' Also, start off on a big site where you'll find games that suit you (ie. Pokerstars
would be my first choice if I was just getting started). Then consider branching out to a couple/few other sites, looking for deposit bonuses, deposit freerolls, etc. to help you build a bankroll online.
Good luck & hope it goes well for you.