How would I say 'what ...' in Mandarin
You can say 我做我想做的. Here I use 我想做的 to represent things that I want to do. Since I use I use 我做, I use 做 after 想。 If I want to say that I drink what I want. I would say 我喝我想喝的。 Here I use 我喝 so I use 喝 after 想 too.

If you want to simplify it, you can say 我做我想要的(I do what I want).