Now any engineering requires lot of MATH and SCIENCE. Depending on the engineering Science can be physics/chem/bio.
Since you are focused on Mechanical Engg, focus, study, and understand Math, Physics and Chemistry more. Since you are in HS you will also need Bio.
Take Honors/AP or other courses in high school and also see if you qualify to take some classes in the local community college for further advanced learning. Many Community colleges partner with High School and let you study and prepare better.
Take SAT and Subject SATs and do well. You can practice them right now and start preparing by borrowing books from the nearby library.
You can also learn a lot on internet/Youtube/other free education classes in all the above subjects to prepare you better.
Get into the college of your choice and work hard and succeed.
find the colleges that have your chosen specialization to apply to.
2Keep getting good grades, & register for co-op
3Get your resume written and start your job or internship search
4Join a Professional Engineering Society or Association
Good luck! You are on a great track!