How to build muscle with a fast metabolism?

I have a fast metabolism, so I just eat whatever for the most part and I stay scrawny. I have been around 120lbs for a long time now, and I’m 5’8”. You can see my ribs. I’m wondering what I can do to help build some muscle – I don’t want a LOT of muscle; I’m not interested in bodybuilding. I just want to be toned, with some definition. Any information would be greatly helpful, but I’m specifically wondering about supplements, like whey protein and creatine powders. Do they help? And do they help permanently, or is it a matter of only helping as long as you continue using them?

Oh, also, any suggestions on what to eat would be great too. And when to eat before/after workouts, how long to workout for, when to take supplements, etc. Basically a crash course on working out. Thanks.