The first thing to do is to condition yourself which means get and stay in your best shape. Do this by working out/exercising and eating right. You can be a skilled or a talented basketball player but you need to be conditioned to play at your best. It also gives you more confidence while playing.
The next thing to do is practice. Practice shooting the ball, dribbling, and doing layups. In your free time when all of your homework is complete and you've studied, go to a gym and practice and play basketball. This will keep you busy after school. Stay away from negative influences and keep with the positive influences. This is important.
The next thing is to attend a professional sports camp, maybe in the summer. They will teach you all the skills you need to become a good or great basketball player. This is important. There might even be scouts there. An alternative to this is to ask your parents to hire a professional one-on-one basketball trainer for you. He or she can help you get good too.
The last thing is to not be afraid to try out for your school's team. You do not need your friend to go with you to tryouts. You can do it. The least thing that can happen is that you do not make the team. But you can be happy that you at least tried. Try out again next year!
Generally, professional sports teams have scouts. These scouts go and watch high school and college games, looking for good players. Playing at a college that excels in basketball would help, as you'd be more likely to be seen, but those schools are harder to get into/onto the team due to competition.
There is also a large component of luck. The right scout has to see you, you have to not get injured, and other factors.
I have no specific knowledge of this industry, or sports at all, but becoming a professional athlete is extremely hard. There is nothing wrong with pursuing this, but you would also be well advised to have a backup plan in case this doesn't pan out.