Take a moment to look at your future goals. If you don’t have any, decide what they will be. Don’t just do a course because your friends are, or your brother is. You are your own individual.
Choose subjects you enjoy. These classes make your life less stressful, but they may be recommended subjects that you need to fulfill for your education goals.
Choose subjects that offer a (reasonable) challenge. If subjects are too easy for you, you will easily get bored and will lack motivation. If they are too hard, you will get frustrated and likely not understand the material well enough to succeed. Take classes that offer you a challenge you are willing to take.
Choose classes that you are required to take. Don’t get too excited picking subjects that you forget the ones that are required for you to take! Pick out your required subjects first so that you know how many other choices you can still make.
Make a list of all the subjects you want to do, and narrow it down. If you need to choose, say, five subjects, make sure you include a couple backups on your list. You probably won’t be able to take every class you really want to take; accept that before you start.
Consult your parents. You get the final say, but you should make sure your parents approve of your selected classes and career path. If they question any of the classes you’d like to take, calmly discuss the matter. Screaming and yelling at each other won’t help you keep your classes.