One warm-up set per excercise is more than enough. If you're going for muscles, you should target 3 sets and 8 reps per set plus one for a warm-up at the beginning (15-20 reps, normal speed (3 seconds per rep)).
(Brown) rice and potatoes are good source of complex carbs, yes. You're carb input is most important in the morning and immediately after your workout. Example: my typical breakfast are cornflakes (very little or no sugar added)/oatmeal and a protein shake. Make sure you get enough proteins in the morning, because you haven't ate whole night and you need some.
You need to eat more than you need because if you don't get enough of other sources, your body will start using proteins as an energy source = no muscle gains. You can't get leaner and bigger at the same time.
Stretching is very, very important. It will help you gain more muscles, prevent injuries and improve your flexibility. First you have to warmup (10 minutes of aerobical excercise - bike, stairmaster,...) and then you stretch for five to ten minutes or so. I recommend that you get yourself a book about yoga and choose the basic excercises and do them before weight training. I don't stretch after every set/excercise, because I find that a bit too much, but I do stretch after I finish the workout for that bodypart.
You said that it differs between different body types? I'm considered skinny, 176cm and weigh only 62 kilograms. Is it easy for me to build up? I'm turning 16 this June.
You're still very, very young and you'll probably gain 10+ kilos in the next couple of years. Remember, you're still growing and people are gaining considerable weight all the way into their early 20s (I'm talking about muscles here

). You shouldn't look at people like Lebron, because they're freaks of nature. Most basketball players gain 25, 30 pounds in college and even more during their HS years. But that doesn't mean you should just lay down and wait. Weight training or any other psychical excercise will make sure that your body stays healthy and develops properly and it will also have positive effects on your mind. I know I'm much more disciplined since I started working out. You just have make sure that you don't become obsessed with it. It has to improve
quality of your life, but it shouldn't become one.
And people, read as much as possible:
http://www.abcbodybuilding.com
http://www.bodybuilding.com
http://www.exrx.net