It depands. You can start with 10 Euros (minimum deposit on a lot of pokersites) and work your way up from there. The best way would be to play for example 1.50 dollar sit and goes. These are played a bit more serieus then forexample the 50cents with 90 people (
pokerstars).
You just need to make sure that you know the everage opponent in that buyin range.
50cent --> could be anything above 67 forexample. (Not great to be in those games).
1.50 --> most will play with 9J or higher. ( these are very playable, I like those personaly the most). You have to play a bit tight yourself but ofcourse you can be very agressive to.