Anyway
I'm here to leave a tip of the day :D
Scouting. Scouting is VERY important. It can change your game drastically.
Example: Say you plan in going mass marines, but you notice your opponent has infestors and banelings, it would be the best idea to switch over to marauders or mech. They don't get blown up as hard when the banelings hit heavy armored units.
Always scout =]