use at least 10% of your monthly income (more money makes more profit)

Or do it like I me and challenge yourself to start with (nearly) nothing. On your first try this can help you to understand the system and learn all you need to know. I started with 10€ and am now up to 40€.

I lost a lot on the way, but now I am confident to handle bigger sums too. When you just see the whole thing as a game it is easier to make failures and learn from then, I guess.