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i'd suggest starting to read books like this.
You can also research and find some great articles about this. But in general, like all careers, you have to start low. Contact a 4th tier team and negotiate to be their youth player's agent. (with minimal to no fees of course). Then, you wait for your first "big" move.
It's mostly about networking after that. You sell a player from your 4th tier team to a 2nd tier team, making lots of important connections. Then you move gradually until you hit a goldmine (a donarumma, ideally)