Do not give up. There is ALWAYS room for improvement. You are way to young to say you already just want to give up. Say you did poorly your first year and a half of high school, but turn around and do moderately well for the rest, colleges will find that very impressive. It is almost like they won't judge you just for those poor grades, that is if you do well and take normal/higher level courses. SAT scores are also important. There are a lot of practice sessions available to help improve your score. Just keep in mind that you can take it as many times as you want. This allows you to be able to send your best scores in each subject to colleges when applying.
Community college is not a bad choice, it actually saves you a lot of money. Yes, you can get into a good 4-year school after that. Just be sure your credits can transfer. However, I think it is a little early for you to be stressed out about college right now. Just focus on the now and worry about the future later. Really just put a lot of effort into your school work and always remember that you can always get help from your teachers, school mates, and tutors.
Being a mechanical engineering major is a very good idea. It is a very difficult major which requires a lot of hard work and ambition.