It is very important to micro manage your units while in combat in Starcraft 2. Being able to improve on this skill will have a direct effect on the amount of wins you achieve while playing.
Common Beginner Micro Issues
It is common for the beginner to often throw their units into battle all at once while continuing to build or check other parts of the map. Stopping this right away will take a player from bronze to silver. Each unit you lose is lost resources, and resources are what matter in this game.
Not to sound like John Madden, but the key is to make sure your opponent is losing more resources than you are in combat. Knowing when and how to retreat is helpful, and moving injured units to the back of the pack during small skirmishes can have a butterfly effect that lasts throughout the game.
Each unit has strengths and weaknesses that you can exploit. Setting up cover with siege tanks or colossi while drawing enemy infantry into range can prove to be devastating if the opponent does not retreat. Likewise, if that opponent counters heavy ground only units with air, you will lose the game quickly. Moving your units in and out of combat at strategic times is the key to winning skirmishes. Easier said than done, but the more a player looks at the micro events within a skirmish as opposed to throwing all units in together at the same time, the better the outcome will be.
Predicting your opponents moves is also an advanced strategy that can swing the game in just seconds. If you scout and know an opponent has reapers early, setting up a trap at your mineral line can completely wipe out his army. Countering quickly, you will certainly be able to take out a depot or barracks.
Common targets that opponents will predictably attack are mineral lines, expansions, pylons/depots and retreating and heavily outnumbered units.
Also be certain to consider the speed of your opponents units. If you are up against mutas with thors, you will need to build faster units to avoid the mutas taking laps around your base while bringing havoc.