Any StarCraft II veteran will know that a couple of High Templars can completely turn the tide of battle. It's also important to learn when you can't win. Protoss players ought to overlap their pylon fields where they are being used to power structures. Zerg players ought to simply center their defenses around their hatchery and resources.
Sealing your base is important if you want to avoid being rushed extremely early. In addition to the suggestions in the video above, it's a good idea, for more advanced players, to try to plug the holes between your resources, and later put a detecting/anti-air structure (Spore Crawler, Photon Cannon or Missile Turret) nearby to scare off potential resource raids. Also, if your economy can support it, it's always a good idea to build several unit-producing structures.
Zealots have their poweful attacks, Marines have their range, and Zerglings have their speed and numbers. Learning to use each of their strengths is key to getting the most out of them. Although they're your basic units, chances are you'll find opportunities to use them all throughout any given game. Zerglings with their speed upgrade make them fierce resource raiders, Zealots en masse can cut virtually anything down in seconds, and Marines are just useful all-round. Be sure to research upgrades for any units you plan to use excessively. A Siege Tank without siege mode is hardly a Siege Tank at all.
Any StarCraft II veteran will know that a couple of High Templars can completely turn the tide of battle. Psionic Storm is one of the best abilities in the game and will obliterate virtually any group of light biological units. Likewise, the Infestor's abilities can be extremely potent too. Most of the best players favor their Infected Terran and Fungal Growth abilities over Neural Parasite. You'll rarely see the Terran Raven unit in numbers greater than one or two as they're primarily defensive, but their Point Defense Drone ability can make your squad a great deal hardier, and their detector status is always a bonus. Using and mastering your race's specialized units will already put you ahead of most of the players in bronze and silver league.
It's extremely tempting, once you've reached your opponent's base, to just start attacking the closest building until it's nary more than a smoldering crater. Don't do it. If you're fighting Protoss, attack the pylons around their production structures, then attack their probes. Fighting Zerg? Aim for their drones and/or their hatchery -- that's where all their units come from. Against Terran, your best bet is to take out their SCVs and then go to work on their barracks. Try to aim for whatever structures are actively producing units. That way, you'll slow (maybe even stop) any sort of interception.
Likewise, ensure that you have your weaknesses protected. Terran players would do well to have a turret and bunker near their SCVs. Protoss players ought to overlap their pylon fields where they are being used to power structures. Zerg players ought to simply center their defenses around their hatchery and resources.
This part is a little more advanced, and it can take some practice learning how to harass and how and when you can use the terrain to your advantage. Catching your opponent in a chokepoint can cause their damage to drop and yours to skyrocket. Putting ranged enemies on top of a ledge can let you attack units on the lower ground from relative safety.
It's also important to learn when you can't win. If you're being led into a choke point, don't follow. If it looks like you might be outnumbered, retreat. Pushing the attack could mean that you lose your army and leave yourself open to a counter attack. Pro games are often a ballet of attacking and retreating and slowly chipping away at opposing army. Mastering that dance, as well as all of the elements we covered above, will put you in a very good position. Will you be a pro? Probably not, but you won't be a total suckball either. We guarantee* it!