I am speaking as being prior service and father of 2 sons who joined. For general military experience at your age, try looking into JROTC. But that is not a necessary stepping stone. The best prep for taking the ASVAB though is doing well in school and taking some of the practice ASVAB tests. You can also find ASVAB books at your local library. The higher you score on your ASVAB, the more jobs that will be available for you to choose from. Have a few ideas of what you would like to do in the military and be flexible as to what branch of service. My youngest wanted to go Army but of the 10 jobs he picked from their list, none were available, so he went Marines. The odds of them having the exact match of what you want to do is very slim. If you're not sure, that is OK too. Go talk to recruiters from the different branches about all the jobs they offer, your high school guidance counselor is also a good resource. If after everything and you do join, if you're in a larger metropolitan area, the Army and Marines usually work with recruits in DEP (Delayed Entry Program) to get you physically ready. I hope this helps a little.
Ron recommends the following next steps:
The best way to gain experience before joining the military is to do well in school (and graduate from High School), get or remain physically active by running or conditioning your body through exercise, and get some experience leading others in school clubs, sports teams, or JROTC programs. Go online to visit the Army, Navy, Marines, Air Force, and Coast Guard web-sites to see all the different jobs available and the criteria you must meet to be accepted in the military services. Good luck!
Mark recommends the following next steps: