Check with your recruiter, but normally you can join your junior year if you are 17. Tell him/her you are interested in the split training option.
You would spend the summer between your junior and senior year at basic training.
When you return, you would drill with your unit one weekend and month. Units will normally let you "split" a drill if you have sports and communicate with them. You would need to go during the week at some point during the month to make up the day you miss.
The summer after you graduate from high school you would go to advanced training, then be expected to train with your unit one weekend a month and two weeks in the summer.
If you get accepted to an academy your academy "trumps" your enlisted commitment. If you subsequently drop out, you go back to the Guard/Reserves.
I'm not 100% positive, but I think if you get an ROTC scholarship same thing. I was actually in the National Guard while in ROTC. When I was commissioned active duty it trumped my National Guard service.
Having prior service is looked at favorably in the admissions process, though not a guarantee.
Also view my thread on the issues my daughter had getting orders assigning her to West Point (she is active duty). I've heard Guard/Reserve soldiers are having the same issue. Guard/Reserve units don't see this often.
Finally, if you are in high school and your Guard/Resrve unit is mobilized for deployment you will NOT go with them. You must first finish high school then advanced training before you can be deployed.