depends what kind of sound you want, but basically a distortion/overdrive, delay, chorus, and wah will give you most sounds.

if you are a newbie, I suggest a multi-effects, a fairly cheap one. (not super cheap like $50, and not expensive like $400! probably something in the $150-$200 range) this will give you something to play around with, experiment with, and learn with. a multi-effects will help you decide which effects you'll want to buy. for example, if you got a multi-effects and you find yourself using delay alot, then you'll probably want a delay pedal, and if you find yourself using a flanger alot (for reasons I couldn't guess, I only use a flanger once in a great while) then you'll probably want a flange pedal... I'd say go with a multi to start. you can get a korg ax10g for $150, it's what I have, and while it isn't the greatest tone in the world it does have plenty effects to play with. and the delays and reverbs are really good actually. or for $50 more you can get a korg ax1500g which is alot bigger and can do even more things.

btw, about the boss OD20, I ordered one, it should arrive tomorrow, I'll let you know how it is when it gets here.