Use a bigger map, with less enemies and fewer phase lanes. Just because a map is designed for 10 enemies, doesnt mean you have to have them, remove half. This will give you time to do things before getting into big battles early on.
If your betting beaten by normal AI's, use Easy's ! Nothing at all wrong with playing easy until you figure out the basics of the game.
Amassing a force, is dependednt on how fast you make money and accumulate rsources. That depends on your research and what you build where. So if your not able to match them force for force, your probably missing a few very key game things that allow you to, or your making bad strategy choices somehow. Its just a matter of learning from your mistakes and looking at what the game has until you find what your missing.