Welcome to CFC, ctwink!
It's hard to give very good advice with so little information, so let me ask a few questions:
1) What version are you playing?
2) What levels do you mean by "higher" and "lower?"
3) By what victory condition do you want to win?
IMO, the best way to get advice on your particular game is to post a save. That way, other players can download it and get a good look at what's going on in your empire.
With all that said, here are some of the most common mistakes made by new players, IMO:
a) Too few workers;
b) Too few roads;
c) Lots of unnecessary structures;
d) Too many defensive units;
e) Not enough tech trading.