you need to expand more. If you are letting the AI get a non-domination victory, especially on King level, you don't have enough cities and most likely you aren't being aggressive enough. Also, you probably are working your cities on balanced, which is a big no-no if you want to be able to play the game well at all. Balanced is slow and keeps you at the same pace as the AI. Go to city managment and start customizing your wokers
try building 2-3 warriors (two workers on two forests)
grow in your capital to 3 population (two workers on two grasslands for five turns)
get your 100 gold settler
Put workers on 2 forests 1 grass for 5 turns, and build a settler.
Grow you cap back to 3 pop w/ 2 workers on 2 grasslands.
In your other cities, start growing them to 3 pop, and start teching Bronze working, pottery, masonary and then irrigation. Before you hit irrigation, rush 2 more settlers, and then settle them after irrigation.
Get to Code of Laws,
Once you are in Republic, start making settlers till you have about 10 cities.
tech to navigation, currency, monarchy, feudalism, democracy.
this should be a good base for you to start with.
Do not build buildings until you are done w/ expansion. An early library is usually a mistake, because a settler will give you just as much science output, because 2 cities can tech just as much as one with a library. Other buildings are generally a waste of time as well in the early game.