How can I start training to become a USAF Pararescuemen?

Look at the standards for the PAST, needed just to get into the INDOC course. You should be able to get almost a max score and then repeat the entire test with a passing score within 8 hours to be able to do well in the INDOC course. That is only the beginning.
https://www.thebalance.com/past-test-requirements-3331760

Start playing sports. They will help get you into shape and also teach you teamwork, dedication, leadership and the ability to overcome hardships such as losing. Sports teams will often provide you with workout programs and if you do different types of sports you get a wide spectrum of benefits. Personally, I played football, swam, and did track and field (throwing and distance). That meant a lot of cardio and strength training. Also, take the opportunity to get your Red Cross certs, and maybe even your EMT- Basic. Extracurricular activities like Boyscouts, Civil air patrol/ JROTC, or explorers (fire/police) help you learn and practice a wide range of useful skills. If you need extra workouts you can find a million SF workouts online.

Do mild exercise, mild weights. Study about emergency medical procedures.

Grow about 6 more inches and gain about 100 more pounds. The special forces don't accept the scrawny.