JAG, regardless of the branch, is extremely competitive these days, and even if you don't deploy, the attitude that you want to will go far.
Unfortunately, none of the military branches will pay for college and law school ahead of time.
The question is: Are you more dedicated to becoming a military officer or an attorney? Remember, JAG officers are officers first.
If you're more interested in becoming an officer, ROTC during college is a great way to pay for it. If you've got good scores and a law school acceptance in hand you can apply for Educational Delay and go to law school, hoping to snag a JAG spot, but that won't be guaranteed. The other option is working for 2-4 years and then applying for the Funded Legal Education Program(FLEP, varies by branch). There's a small number of active duty officers selected to attend law school with a full ride from the government and continue to be paid while in school. It's a great deal if you can get it but you'd generally need to have a Tier 1 acceptance in hand to even be considered.
The bottom line is, there are many ways to serve and paths to becoming a Judge Advocate. Depending on your interests and branch, you have a few routes. But you absolutely have to be prepared to be an officer and serve in a combat zone.