Stick with the F15 first and do not move on to any other plane until you have mastered it. The reason is that the F15 is a very simple plane and the concepts of air to air combat are much easier to learn in the F15
Learn the F15s systems and know them like the back of your hand. Especially the radar. But don't just learn how the f-15s radar works, learn how all radars work so that you can take those Concepts and fly any plane well.
Then, I'd personally move on to learning air to air combat concepts. The two main concepts you will need to know are 1. BVR (Beyond Visual Range) fighting and 2. dog fighting.
BVR fighting itself encloses concepts like notching, cranking, etc that you'll eventually learn.
Here's a guide I made a while back which will show you the basics of the F15s radar, missiles, and one or two other systems:
https://docs.google.com/document/d/1i0dIEqC5xR4D7hJAcrOqpNr66E2x_06Z0cVt9drneqU/edit?usp=drivesdk