First off, try not to be too hard on yourself - heh. You will get better the more you play. Check out this site's War Academy. As many people will tell you, there are a ton of good pointers.
While hagbart and petey are correct in that you need to expand, you also must keep your unit size up. When an AI comes to you and demands tribute, they usually believe that they are stronger than you. You refuse, then find yourself having all of the civs declaring war on you.
This was usually my biggest problem, starting off. The reason for this, I found here at CivFanatics, was that the AI counts up all of your units (military, worker, and otherwise), compares those unit figures to its own, then decides if you are an easy target.
So, while expansion is a very important aspect, you must not forget to maintain a balance of growth and military strength.