If follow banrkoll management , for SnG enough $10 this 100BI 360p $0.10 , for cash 25$. This course if you know the basic concepts of the game, and so the more the better.
Depends on what you want to play. One table SnG's require a minimum if 70 BI, Cash Games around 40BI and MTT's 100BI. These are usual requirements, but some people say more, others say less.
Are you willing to redeposit? If so then you can play higher than if you never want to redeposit.
Are you playing for entertainment or are you planning on making extra income? For entertainment just deposit whatever you are comfortable losing. If it's for income then you have to be more rigid in your bankroll requirements.
Finally and probably the first question to ask is are you a winning player over a large sample size? If not then and you are just starting out then I'd just deposit what you are comfortable losing and play the lowest limits because you are going to lose this money. Consider it training money until your game improves.
If you are just starting out, I would make a small deposit and play in some of the micro cash tables or .50 cent to 3 dollar range tournaments. 25-50 dollars should keep you entertained for awhile and at least get you some experience. Expect to lose most of it if you haven't played much. There are a lot of free rolls on certain sites as well you should check out. Good practice (altho people play in them a little looser) but you can still get a feel for what hands to bet and fold.