Creatine

Yes. Monohydrate is the most researched form and usually the cheapest by a clear margin. Newer forms cost more without a real edge for most people, so unflavoured monohydrate in water or your shake is the simple place to start.

Labels typically suggest around 3 to 5 grams a day, taken every day, with timing that isn't fussy. Some people load with more in the first week to fill their stores faster, but that step is optional. Being consistent matters more than the clock.

It draws a little water into the muscle, which is where the fuller look comes from, not fat. Any early jump on the scales is usually that water and it settles down. It suits almost anyone training for strength or size, and the unflavoured mixes into anything.