StubHub is worth a look. Given that Jax sucks you may be able to get tix at the stadium box office as folks turn them in. Never been to the stadium so no help there.
Since it is your first NFL experience, get there a couple hours early. Again unfamiliar with tailgating rules in Jax (poking around the net should get you some info) but in general a large part of the NFL experience is tailgating before (and after to much lesser degree) and also a lot an ancillary events going on before the game. Sponsors often have promotions and giveaways. You can walk around the stadium and take it all in, both outside and inside. It's not just a game, it's an event. Expensive event lol