After playing this game intensely for about a month and watching replays of high caliber players, I thought I'd share some helpful tips for newbies out there looking for a way to get better at the game quickly. These are some of the learning lessons I've had that I can pass on to you.
Unit advantage is extremely important. I'm not talking about unit levels or counters (although that is one aspect), I'm talking about creating situations to keep your units alive.
Tip #1 - 2-on-1s and trading
Try to create favourable trades as much as you can.
The easiest way to do this is by setting up situations where you end up in a 2-on-1 against your enemy. For example, let's say at the beginning of the game you decide to make Riflemen right away. But, your opponent makes Shock Troopers. That's a bad trade for you. What you should do here is retreat your Riflemen, build a counter for the Shock Troopers (Flame or Shock Troopers, Buggy, etc.), and then attack with both your Riflemen and your counter. This will reduce the amount of damage your opponent does because the Riflemen are helping out. Even if your Riflemen die, your counter will maintain more health. This creates value. Essentially, you are trying to extend your Tiberium investment as much as possible. By winning the value game, you are more likely to win games because your opponent is spending more than you are to overcome your defenses. Also, having more units on the map means you are able to capture more zones.
Another situation I see a lot is where you hold two zones, and your opponent decides to attack you at one of them, and you can create a favourable trade by moving the unit from the other zone to help the unit being attacked, but decide not to because you don't want to lose that zone. A lot of newbies make the mistake of leaving that unit there, and keeping it as a 1-on-1 trade. You reduce your chances of keeping that zone, and losing that unit by not supporting it with your other units. Granted, if the missile is about to go off it doesn't make sense to do this, but if the missile isn't, you have time to recapture that other zone so it's usually best to move that unit.
Tip #2 - Know your best, cheapest counters
In the beginning of the game you are still becoming familiar with all of the units in the game, and trying to understand what each one does. There's no way to learn this by playing, so keep practicing. But, keep a focus on counters, and figure out which units counter which the best, at the lowest cost. For example, Missile Troopers are excellent against Predators and Scorpion Tanks. Being Infantry they take less damage and deal out much more damage. At a cost of 20 tiberium versus 60, it's an excellent trade. Or, Flame/Shock Troopers against Zone Troopers. That's a 40 versus 120 tiberium trade and the Shock/Flame troopers will come out on top. Try to maximize your value!
Tip #3 - Move your harvester when you get rushed
This is a bit more of a basic tip but if you get rushed, move your harvester! Even against Jump Jet Troopers, if you move your harvester towards your base and position your counters well (Riflemen/Militants work well against Jump Jets by the way), you will mitigate your harvester getting destroyed and fend them off. If you do this right, you'll have put your position in a tough situation where they are low on tiberium and you are not, allowing you to create early pressure and win.
Tip #4 - Deck Building doesn't always require high end units
While it seems correct to always put high cost units in your deck so you can survive the late game better, that's not always the case. Using the logic of value trading, you don't always need to have big units to win. In fact, many top players don't use any big units in their decks. You would be surprised at how powerful a Predator can be at 60 tiberium. You'll probably notice that a lot of games are won before big units come out. So, don't feel compelled to put a bunch of big units in your deck. Try it without, or even just one. I think you'll be surprised because of the extra options you have in the early game.
Tip #5 - Spend gems on rushing crates, not buying them in the store
While it seems enticing to pick up an Epic Crate at 600 gems, this is not a great way to spend gems. Instead, spend those gems on rushing crates. At 24 gems per rush, you have a chance to pick up an epic crate anyway (though, the chances are not always high, I've managed to secure about 5 of them in the past month of playing). In the long run, it's a much cheaper way to gain credits and cards. Even if you get a common crate, you still have a chance to get Epic Cards in them which is a great value.