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.
If you mean what is the smallest you can buy-in for and hope to build, I guess $10 is okay. Ideally, you would play freerolls and small Sngs to build that up. If you prefer cash games, then you should start with at least $25 to have a chance to ride out some of the waves (variance can be a beast). Check out some of the twitch/poker school videos on playing micros if you are going to go the cash games route. There are a lot of good players with good strategy tips out there.