Resources:
Take note of your momentary balance of income / outgo before queuing up some big project. It's really easy to find yourself "in the hole", as it were, early in the game by simply choosing one specific item or another. For instance, metal storage takes a lot of metal to create so your metal outgo will spike heavily compared to your energy consumption. Find yourself in a position with high outgo and low income and building will slow to a crawl. Some maps are intentionally stingy with metal so they take some different strategy with respect to resource procurement and how you spend.
Don't waste time and resources on wind turbines unless you know the map has high wind velocity. If you're not sure, just build a single one and check back on it from time to time since its output varies with the ever-changing wind speed (hmm, a parallel to the real world?). Wind turbines are also easily destroyed so you're usually better off with solar collectors if there is any concern for an attack on your power gen areas. If the map has meteors I avoid wind power entirely.
I like to get at least one metal and one energy storage building built early on, since those can really make a difference in production speed if you queue up several projects at once, using the stored reserves to supplement your resource income. At the end of a match, you'll have a screen totaling how much unused metal and energy you collected during the match. Those are completely wasted unless you can store it to use it as needed.
Units:
Air: I like to get an airport going fairly early for a couple reasons, even though it's fairly expensive. First, scout planes can uncover a lot of the map in a hurry; they also have radar so they're useful to create a flying radar "picket" for early detection, using the patrol function. They're cheap enough that it's not a big deal to lose a few to enemy fire. Second, I like the flying construction bots since they can get to far away places faster than your other construction units. This makes them handy for snatching metal deposits far from your base and for setting up radar sites. I also make one heavy lifter to move the Commander around quickly - handy for escaping an assault (be careful he doesn't get shot down) and for exploiting areas with low enemy presence.
Commander: Besides his superior construction speed, don't forget his special abilities - capture and D-gun. Capture lets you, well, capture stuff. The cool part is that you can capture your away into the opposing tech tree. Use your capture beam on an enemy construction unit and now you can use that newly-acquired unit to build an enemy kbot lab or vehicle lab. From there, you can get everything the enemy gets! A couple notes about the D-gun: it sucks a lot of power so be careful using it early on when your energy income is low, and using it on an enemy Commander will end your game since his explosion radius will take you out as well. The AI doesn't really care about this so I'll retreat my Commander if I encounter his. If you aim carefully, you can use the D-gun to take out submerged vehicles that are close to shore. You can't fire it when underwater, though.
When you start to build level-2 facilities, the Commander can really speed production of those buildings by contributing to the project. For instance, if you have even 3 or 4 aircraft building the level two airport, it'll take forever. Add the Commander and it'll be finished much faster.
Construction bots: Not much to add here except that you shouldn't forget the recycling capability mentioned by DrWevil above. Note that it works not only on scrap (blown-up stuff), but also on fully functioning structures and units. Found a metal deposit with an enemy extractor already built atop it? No problem - recycle the extractor and build your own! I'll get into it a bit more later, but you can also use them to augment the construction capability of other units and factories - adding more construction units to the task will speed up completion of the job.
Ballistic units: not great against other units since the missiles are quite slow, but they do wonders against enemy buildings. 3 or 4 ballistic launchers can make a hole in the enemy defenses from a safe distance in short order, and will also trash a production building in one or two salvos. They don't need LOS so they work well hiding behind hills and the like. They also move slowly (except amphibs) so keep them away from firefights.
Peewee (small, weak, quick scout-types): Use them to scout, of course, but also to harass the enemy construction units and metal extractors. After the Commander and your first couple construction units, these are among the most important early-game bots.
Building things:
Take advantage of using multiple units to speed up construction. This goes for building, say, a geothermal plant as well as supporting an Advanced Kbot Lab. Set a couple construction bots to "guard" a factory and you'll speed up your build queue of combat units a pretty good amount. Just remember to watch your outgo of resources.
Map stuff:
Gotta find that metal early. Energy you can make anywhere with solar collectors and fusion plants but decent metal collection relies on deposits (except Core Prime maps).
Many of the maps use choke points so take advantage of that for early defense. Conversely, use air power to get around that problem by lifting your construction units past and around the blocked area.
Consider LOS and terrain features when determining your defenses. Putting a medium-range cannon behind a hill wastes its capabilities. Likewise, placing it near the peak of a hill helps the round pass over those ground obstructions at lower levels.
Naval warfare:
Naval units have interesting capabilities and built-in limitations, with more of a rock - paper - scissors aspect than the ground units. For instance, certain ships can't retaliate against subs or aircraft, and subs can't attack amphibious bots (excepting the couple 'phib tanks that drive underwater and not on top of it). Subs are super-slow and can't absorb much damage but they can also quickly deep-six construction platforms and resource units in the water since the torpedoes do heavy damage. Early on, the light cruiser (from the level 1 ship platform) is a good bet since it can handle surface and submarine threats; pair them with the level 1 scout ships for some air protection. The bigger ships are big and slow targets so they can be quickly overwhelmed by air units.
Keep in mind that combined-arms is pretty important in the game. For instance, you want to sneak some subs into the enemy shipyards but the sonar stations are providing detection for the torpedo launchers that are blocking your way. No problem - those structures can be knocked out easily with air power, paving the way for your subs.
