well, if you are just starting off, i recommend reading the manual and all of the guides in it
then follow the terran guide in the manual, and do the first few missons, and that will help you with your empire economy stuff, which needs to be mastered before you go out and start fighting
and you can fight anyone you want! (technically) there are restrictions: DP, NP, attack ranges etc.
you will learn about that in the manual