Well first, you need a computer.. Then a mouse.. Then a keyboard.. If you got those basics covered, there's a few more things you need..

1) Time - Lots of it, it's engulfing.. It's like a blackhole.. It's an addiction.. It's fun.. First start with storyline, then try playing some multiplayer games..

2) Practice - Fine tune the basics, watch http://day9.tv/ for pointers, it'll be useful to you.. If you've got friends playing with you, even better, share info and whatnot.. Lurk in http://teamliquid.net/ for more info and discussions and strategies.. Learn basic strats like 2 rax or 4 gate or 7 roach rush..

3) A hot girlfriend - Nothing to do with starcraft, but it's always important to have one

4) Introduce said hot girlfriend to me - So I can impart my so-so starcraft knowledge upon you.. For the low payment of said hot girlfriend

5) Fap, fap, fap, fap, fap, fap, fap, fap, fap, fap, fap, fap, fap, fap, fap, fap.....