How do I build my credit back up and clean it at the same time?

So I am I need of a car my goal for the next month or two is to save an buy a nice little used car. I eventually want to get something really nice tho. I happen to work at a car dealership.

My issue is I previous financed a car and it got repossessed it was just too much for me to afford at the time. That partly destroyed my credit then on top of that I recently co signed for a house for my mom along with my brother. But my mom hasn’t been well in top of things and that’s also been destroying my credit and my parents have been putting my name on things and basically destroying my credit before I even knew what credit was.

So now with the repo and everything I can’t lease or finance any cars and I eventually want to get an apartment which I won’t be able to do with my credit the way it is.

How do I clean my credit and build it up slowly over the next year and if I can’t really clean it how do I at least start building it back up?
How do I build my credit back up and clean it at the same time?
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