No matter what, dont let ur guard down, and keep on making units. At first, you should first start making some marines about 15-20 and keep them around ur base.
Then, once you make a factory, you should start making some tanks. Generally, tanks are one of the most powerful defense units when in seige mode, but are vulnerable to many things... like air units, dark templars, etc... so don't just use tanks as they are completly helpless themselves, but with some marines in bunkers, 2-3 missle turrets, your defense is going to be tough to beat. As you advance a bit more into the game, start making goliaths...and vultures with mines upgraded on. Once thats done, set mines all over the map so you can cleary see what is happening on and on. Then make some dropships about 7-8 of them. Lure ur enemy into ur defense, and kill thier units. Then use dropships and send tanks and goliaths into thier base. Knock out thier miner gatheres, thier remaining units, then when they are completly vulnerable, attack using ur goliaths thier buildings and you will have a good chance of winning.

Remember:this does not ALWAYS work, but about 60-70 percent it does...depending on what ur opponent is up to.