Depending on your plane's capabilities vs. your opponent, try climbing at full afterburner. If you fly something like the Phantom against MiG-17s, the MiGs won't be able to keep up. If they try to gain altitude like you do they'll slow down a lot. You might be able to dive in for a kill then.
Also, set everything to "easy" until you start figuring out how you can best shoot down enemy planes.
I've been playing Strike Fighters for a few months now. I didn't realize when I bought it all the mods available, it just looked like a good game!
I haven't done any modding of my own, but I might have to try now that I've got loads of aircraft and weapons to play with. Sure is fun using a nuclear AAM and slaughtering whole squadrons of enemy bombers at once!