What I Want From You 〜てほしい

The person you want something from is marked with に, not は or が. This lines up with たい, which you already know describes wanting to do something yourself, while てほしい flips the direction: someone else is the one doing the action, and に marks who that someone else is.

ほしい conjugates like any other い-adjective attached at the end, ほしくない for the negative, ほしかった for the past, ほしくなかった for the past negative, exactly the pattern you already learned for 欲しい itself.