Technically very little — most brokers, Exness included, let you open an account with a small deposit, and cent accounts let you trade with a few dollars. Realistically, the size of your first deposit matters far less than whether you can afford to lose it entirely.
A sensible beginner approach is to spend the first month on a free demo account, then start live with an amount whose total loss would change nothing important in your life. Small accounts make small money — the point of the first year is learning the process, not the returns.