Join AFROTC, get good grades, do well within the ROTC unit, and stay fit.  Once you can choose your AFSC, pick pilot!  If you have done well in college and ROTC, are medically qualified, and get a pilot slot, do well at UPT.  UPT is where you will track select (heavy, fighter/bomber, helo, etc.)  After you do that, you will go on to further training in your track and can (if you have ranked highly enough to get your choices) choose an F-15 (if one is available).