I have gap between my shoulder and collar, what exercise I should be doing to fill the gap. In other words, I don't want my collar bone to be shown, how to cover them with muscles.
There are no muscles that directly overlay the collar-bone, so you can't really cover it. Poking around online, it looks like this isn't an uncommon question. The general solution is to build the muscles on either side, which would be the upper pectoral and the trapezius muscles. As for what exercises to do, upper-pec is generally a matter of decline pushups (feet raised) if you want to use your bodyweight, incline benchpress if you want to use weights. For the traps, it's generally shrugs, handstand pushups, and arm circles to build them up.
It’s important when you are prioritizing body parts to continue working supporting musculature. If you don’t, you run the risk of creating an imbalance. In your case, that might mean working the deltoids and upper chest, as well as, the trapezius. One of the better exercises, in my opinion, for pectorals is the dumbbell pullover. It’s a somewhat forgotten exercise that works lots of supporting muscles. I’d also recommend an incline movement like incline dumbbell press which work the pecs and the clavicular head. Lastly, you should consider working all three heads of the deltoids to balance out your upper body. Something like the Front Raise should help the Anterior head.