Bad. I don't know how to live man.
I don't really have much hobbies or friends. So aside from school work, all I really focus on is my nutrition and weight-lifting. Which has been working, but has been stressful (especially since my overall body shape is similar despite the loss in fat and increase in muscle).
I wanna make music but I can't play any instruments. The family doesn't have much money. So I'm short changed on being able to experience simple pleasures sometimes. I would get a job, but I have SAT's to study for - which I haven't been doing at all.
I just feel so empty and bored sometimes. I don't know how others have good grades despite having so much to do after school.
I'm sure you have some group of people that you usually talk to in school. You can expand your circle by finding out what other people like and finding common ground with them. Get involved in your surroundings. Once you have some circle of friends (even if it's not a particular strong friendship) start hanging out with them. If it doesn't happen on it's own - initiate. Find some activity you and others will find fun.
If you're seriously into making music, you don't need to be able to play an instrument, you can learn. Anyone who knows how to make music had to learn it at some point. You can look for an instrument you'd like to play, maybe watch some vids. Go to a music store to try some instruments with your own hands. The guitar is a popular choice. Not all instruments are expensive. They're not really cheap either, but you can save money to buy one if you're serious about this. I don't know how the prices are like where you live, but a simple guitar, for example, won't cost more than an average smartphone. Making music on the computer is also an option, there are various softwares for that.
About the SAT's.. you should really take it seriously if you're thinking about going to college. Find your motivation, it won't work otherwise. And that goes for everything you do.