You can use products like bio oil to help the skin heal, but the stretch marks will never go away. You can't shrink them, you can only make them fade.
If you got these stretch marks from gaining muscle, you are gaining muscle too quickly. This will end up giving you half *** muscles that look good but don't preform well, and that will shrink the moment you stop working out. Gain muscle slower and more deliberately, that way you will actually be strong and will keep the muscle.
If you got these stretch marks from gaining weight, lose the weight. Simple as that.