Depends on what you want. If you're going for MTT's or SNG $20 is plenty enough if you're short budgeted. You can find $0.2 and $0.10 buy in at
pokerstars if you're outside US.
If you want to play in cash games I'd like to say maybe $50 is the minimum and that is plenty already. if you're gonna play $0.01/$0.02 stake in Pokerstars that should be enough.
First, invest your
bankroll to improve your Poker skills rather than building your bankroll, and if that happens I'd guarantee you a successful career.
Learn more by watching videos about poker, reading books, etc.
Good luck and don't forget to have fun!