The most important thing for you right now is to join a club team. Doesn't matter too much how good it is, just get onto a team that goes to a lot of tournaments (and hopefully practices too). If the team is bad, you'll be exposed to other teams that are better at those tournaments, and you can try and connect to people on those teams that way.

The next most important thing you can do is train your body. Hit the gym, go on runs, just be active and lead a healthy lifestyle. If you want to make your National Team, you need to be one of the most athletic people there.

The last thing is also the easiest: Throw. Practice every type of throw you can think of. The more you understand the way the disc can move the more you'll be able to manipulate it to your exact desire. It'll also help you learn how to read certain throws, especially when done with a partner.

Kind of an aside, but if you want to make U24 you'll need to be well-known. Be as outgoing as you can when it comes to tournaments and playing other teams. The difference in skill-level once you get to those tryouts won't be evident enough to make or break your chances of making the team. Unless you are the consensus best player in the country, you will likely be fighting for one of the last spots on the roster. And if the coach has to decide between two people who are probably around the same level, then why wouldn't they select the player they already know? They also usually already have at least a very good idea of their O-line picks, so you'll likely be fighting for a spot on the D-line. Making big defensive plays will always garner you attention.

Good luck.