I just contracted on my first house, when can I start cleaning and painting it?

Yep, you have to wait until closing, unless previously arranged with the seller. You also want to wait in case something comes up, i.e. appraisal comes in low, inspection may reveal large defects that could void the contract, survey problems, etc.

If your deal falls through, any improvements you've made will be forfeited.

You definitely need to wait until after the closing. Unless a special arrangement has been made, you won't get the keys to the house until you are at the closing table. You also have to have a home inspection which could turn up some major issues that need to be negotiated with the seller as far as you fixing them and the seller lowering the price or the seller fixes them before the closing date.

You definitely have to wait till closing. You don't own it until then.

Yes, wait until closing. You never know what glitches may arise.