You have much more time than I did, then. I was nearly 28 when I started the process.

Again, I'm not a guard pilot, nor in the military, and there are plenty of people on this forum who fly both for an airline and the guard/reserves, and they would be able to give you the most accurate and reliable gouge on the different ways you could tackle your goals.

With that said, if it's what you want, there are some freely available resources online where guard units post there upcoming UPT selection boards and they include contact info and details about what needs to be in your application package. I had even cold-called a couple units via the contact info they provided. Results were mixed; some units always picked up the phone and were very happy to answer my questions. Other units I never heard a peep from via their posted contact info.

Airline jobs will always be there till you're 65+ years old. The time frame however to go from hired to in training at a guard unit takes quite a bit of time; maybe consider applying sooner than later to guard units you're interested in.

Finally, it is never too soon to start networking, be it for a civilian pilot job or military. Try to find out among friends/family/co-workers if anyone knows a guard pilot with whom you can get to know; it could possibly lead to an internal recommendation if you're clever. I was fortunate that my dad had a client who was a guard pilot at the unit I was most interested in and was able to capitalize on that relationship.

edit to add, as for general guard info, there are tons of threads on this forum discussing that stuff. I'm sure a search will turn up a lot of results on that.