How to level a tilted free-standing closet
Unless you want to drill holes in it and anchor it to the wall,  using wood shims (very thin pieces of wood) would be the best thing.  That is about all you are going to be able to do.  You can buy shims at any hardware store, and they are thin enough that you will be able to go up about 1/8 inch at a time until you get it level.  they do the same thing with pool tables in your home too, so I think that this would be the easiest solution.