That's the deal... you need to have a plan. Of course, plans change, and no plan survives contact with the enemy, and all that. But you still need a plan.
I start with my plan depending on my start - the starting position and the leader. If I see that my capital site is on floodplains, with Wheat, Pigs and I am a Philosophical leader, I'll tentatively plan to go heavy on Great People. If I start near Gems/Gold, I recognize the commerce bonus and make use of it by founding a religion first.
Also do play random leaders - it will give you more perspective.
If you go to war, also have a plan. Have a clear idea of why you are at war - do you want to conquer city X, get access to resource Y, or do you just want to slow down the development of that particular civ? Be flexible. If you see you can't take their cities, but have a superior field army, pillage the hell out of them. Pillaged Villages and Towns HURT.
Oh, definitely get a couple of friends. It's much better to have some civs Pleased with you, and the rest hating you, than to have everyone annoyed or cautious. If you try to make everyone happy, you'll fail, and end up with no friends. So have some friends and use them!