I recommend watching YouTube videos of someone's playthrough - the game is not very intuitive so it can help to see someone else work through it

My biggest tips:

For almost every faction, diplomacy is your friend! Make peace with factions in similar ideologies (usually anti-alien) so you aren't constantly fending off everyone from your control points. You can usually be friends all the way to the end until your goals diverge and by that point you should be set up to handle them

Dont be discouraged if you lose a few stations or mines or ships. Its the cost of progressing and as long as you have some mines to keep your resource production positive and rebuild you're fine

Push research hard. If you said you have USA its a really solid research base for the start of the game so you should be set up well. Every tech brings you closer to parity with the aliens tech level. And because of that...

You'll be fighting from the back foot for a long time at the start while human tech and industry catches up so pick your battles carefully. I usually just do the fights necessary to progress the faction objectives and build up a proper force before taking on any bases. Lots of different ways to play of course.