I've played a bunch of matches in the practice league now, and I continue to get pummeled in nearly every match. But before I throw in the towel, I figured I'd ask for some tips first. I'm clearly doing something wrong, but I can't figure out what. 
 
http://cid-36266574363dc0bb.office.live.com/browse.aspx/.Public/SC2%20Replays

I've seen the 3 first minutes of the "metapolis" match and can already see why you'll most likely lose that one. 
 
You didn't build a supply depot.
 
Getting supply blocked and then not building a supply depot for the next 2-3 minutes is really bad. This means that you won't get any new SCVs in the near future, which is a big deal. Also getting a so early Refinery is pretty useless since you won't be needing any gas at that point anyway. Spam SCVs untill you can afford a supply depot while still producing SCVs (usually when your supply limit is at 9 or 10), and then continue to produce SCVs untill you have 12 or more and THEN place your first barracks and/or refinery.
 
Also, watch this.

Watch replays, read build orders, read up on strats, get use to hotkeys, and work on your speed. Just playing practice matches won't do much if you have no direction.

Just watched the first game on deser oasis, here are some flaws I noticed.
 

• Left SCVs doing nothing when they should be mining/building/repairing. Having your SCVs just sitting there waiting is a big blunder in the early game.  A tip is to queue up commands, once giving the command to build a supply depot, then hold down left shift, and click on the next thing you want the SCV to do e.g. mining.  This can be done for any unit/command in the game and queing commands is very valuable.
• You went for the engineering bay far too soon, in the early game you need more units than you need the damage/armor upgrades
• Made two refineries way too soon, and then you only had 1 SCV on each.  You should make your 2nd refinery when your first has 3 SCVs (more than 3 and your extra workers will just wait for the SCV inside to finish mining), it is quite bad economy wise.  A standard terran opening is to build 1 refinery after the 1st barracks has started building, and the 2nd refinery when you start building a factory (or at 3rd barracks if you're going for a mainly bio composition)
  
• You didn't get supply blocked (which is good) but you over compensated with supply depos when you didn't need them yet. Learn how much supply each unit takes up, and build supply depots when you think you're going to be hitting max supply.
  
• No scouting, no use of the xelnaga watch towers, granted this is hard to do in the practice matches what with the rocks in the way.
• Because you didn't scout, you had no idea that your opponent wasn't going for air at all, making the missile turrets a waste of time (though once the mid game rolls around I do recommend setting up missile turrets at each expansion)
  
• You always want to be making SCVs, an aid in doing this is to hotkey your Command Center.  You hotkey buildings/units buy holding down cntrl and pressing a key from 1-0.  For example, if you hotkey your barracks to 1, you can press 1 and then A to produce a marine, double press that and the camera will snap to the barracks. This way you can be looking at whatever you want, such as your main army, and still be producing units without having to go all the way back to your base and then all the way back again.  I reccomend hotkeying all unit producing buildings (and army units). All commands have a hotkey, and saves valuable time insead of moving the mouse all the way to the bottom right of the screen (side note: if you have an army of tanks and marines in the same control group, press tab tos witch back and forth between units for example press D (or E i'm having a complete blank right now but it's one of those) to siege your tanks, and still have control of the whole army).
• Check on the internet about "walling off" your base, this is invaluable for when zerg/protoss are going for a zergling/zealot rush.
• You also made higher tier structures (factory, ghost academy, starport) before you didn't have much of an army to begin with.  As a beginner you really need a core army before you get to grips with what each of your units can do, and then you can move onto funkier stuff.
  
• I also suggest going into each match with a predefined plan that you can say in one sentence (stealing this completely from day[9]), like going for mass marines and siege tanks, or marine medivac marauder composition.

• Aaaaaand, never let your minerals/gas get over 200, unless you're saving up for something. Always be spawning!