I'm under weight for my age, but my BF % is heigher than I want it to be.
To get firmer, I want to gain muscle. Can someone please explain the whole muscle building process of eating more calories than you burn? How can you do this with out gaining fat? How should you weight train? How long? When? Cardio? Tell me everything I need to know.

You will gain a little fat if you want to gain weight.

Theres only two ways of doing this, gaining weight (and fat) and then cutting away at it (and burning the fat).

Or getting rid of the fat first and then gaining weight (and fat) while working-out/training so the fat is kept low as you build muscle.

Either way though youre going to wind up putting on a little fat while you gain weight. Its fairly impossible not to gain fat while gaining weight unless all your meals come from an experimental diet lab or some such nonsense. Foods that have protein usually have fat and one can not live on egg whites, whey and broccoli alone.

Everything you need to know is probably already on these forums, and you have to search the information. There are many slightly different approaches in working out and nutrition.

The basics: Consume more calories than you burn in a day. Aim for a 40/40/20 split of carbs/protein/fat. Strength train and do cardio (how much of each depends on you, your personal goals, and what people think are the best methods). Don't over-train and get plenty of rest.

How much is a little fat? Like one pund or five?
Also, can you explain why you need to eat more calories? I've gained some muscle just training on my fat loss diet of 1500 calories. Not a lot, but still I don't want to be huge, I just want to have a better body fat %, and if I keep doing cardio like I was doing when loosing fat I'll loose muscle too and just become some skinny kid. What's the science behind eating more calories? It doesn't make sense to me that you would also do cardio with bulking. Why eat those extra calories if you're just gonna burn them in cardio. Can't you gain muscle just eating good food with good fats and lots of protein while lifting weights and just being active? I'm confused.