###The first thing which you need to do, is (to) clean your room.
The first thing which you need to do, is (to) clean your room.
- The first thing is (to) clean your room. Pr.Clause
- Which you need to do. Sub. Adj.Clause. modifying thing.
Together it's a complex sentence. Noun can't qualify pronoun attributively. It can at best have its renaming predictively.
