Beginner's Tips and Tricks Guide on Steam

^Agree with both previous posters.

A few other things I've learned along the way:
*Early game research matters tremendously. The difference between picking the "right" and "wrong" upgrade for the situation at hand can very easily lead to you losing quickly, or beating the whole game. As far as weapon selections are concerned, I find that you really can't go wrong with a projectile cannon and energy cannon to start, with the first upgrade for each being burst.
*The last level is a game unto itself. Most commonly when I lose, it's in the first 5-15 turns, or on the very last level. This has taught me the importance of treating the last level differently; what worked up until then probably won't work here. Specifically, plan for losses and save some scrap/research so you can purchase reinforcements.