As someone above me said, it's all about the macro. Macro your buildings to hotkeys so you can continue production without your attention being focused on your base all the time, such as training more units while you attack. I would suggest sticking with Random so you get to grips with all three races. Being random also has the advantage of briefly masking what race you end up with so the enemy doesn't know which build to take immediately. For example, i play Zerg, and other Zerg players tend to go full muta-ling rush at me, but may attack differently if they thought i was Terran or Protoss. It can really help to throw the opponent off balance. However you will find you gravitate towards one particular race, and it is that race you should just focus on and get good at. Don't play Terran if you suck at Terran but are great at Protoss. It's perfectly normal to be good at one particular race. Make sure to constantly build workers, even if you're saturated. You can send them to your expansion later. Don't climb the tech tree too fast. So often i've seen someone go from tier 1 units up to the heavy duty stuff and spam upgrades and end up losing to a cluster of really weak units because they didn't have any real army to speak of. Upgrades should come when you have a sizeable army and money to blow; the exception being for Zerg. You may want to get Burrow immediately, for example. Try to harass your opponent and stop him from expanding. If he gets too much map control, you will be boxed in and it will be impossible to win. Finally, scout, scout, SCOUT! Knowledge is power, especially in a game where you are relying on your information on the enemy to come up with a suitable counter. Are they building aerial units? Then get some marines and vikings out there! What about strong ground units? Set up some siege tanks in strategic points around your base!
Lastly, don't worry about getting your arse kicked in the first ten or twenty matches in the practice league. We all went through that and it's how you get better. Make sure to save and watch your replays, especially from both perspectives of you and your enemy. learn where you went wrong so you don't make the same mistakes again. Focus on getting your Actions Per Minute (APM) as high as you can.