Gameplay dos and don'ts
Do get all the metal you can
Metal is needed to get just about anything done in the game, and you want as much of it as you can get. Build mexes and reclaim wreckage from the battlefield like your life depends on it (because it does), and do your best to keep your enemy from doing the same.
Do use your units
If a combat unit isn't shooting, it isn't paying for itself. If an opportunity to profitably deal damage to the enemy arises, take it. If your team is engaged in combat somewhere, send your units to help out. But don't throw your units away needlessly.
Do build energy resources
Unlike metal, energy is consumed a lot more, especially if you have shields/cloaking units, so the best thing to do is to make a lot of energy creators, next to metal extractors. You should always have more energy than metal, or as more than you can spend. Excess energy is used to overdrive metal extractors. Because overdrive loses efficiency as more power is pumped into a metal extractor, it best to have more mexes on overdrive. Be warned, if you are using anything else that isn't a solar panel, they can explode and severely cripple your economy.
Do have mobile defenses
Despite towers being cost efficient and powerful, it just isn't enough to have just towers. Make mobile defenses as they can move to many places, and react to any raid or push and it will make sure your not a sitting duck to an artillery raid. The best ones for defending tend to be raiders as they are very fast and can react almost immediately.
Don't make lots of defences in one place
The enemy will simply go around to where the defences aren't (like where your allies are), and to fortify all possible attack points simply isn't economically feasible. Seed moderate defences where necessary (e.g. at chokepoints), sprinkle light defences elsewhere to deter raids, and use mobile units for the rest of your defensive needs (or better yet, use them to be the one doing the attacking).
Don't send your units in one by one
They'll just die one by one. You want to concentrate your forces wherever possible; see Lanchester's laws for why.
Don't make lots of storages
You want to be spending your metal and energy as fast as you can, not stockpiling them. Storages should only be made under very specific circumstances, usually in FFA matches.
Don't make more nanoturrets than you can afford
Nanoturrets (Caretakers) can help your factories and constructors build faster, but only if there's actually metal available to spend. If you only have enough ingredients for three cakes, having ten ovens won't make them bake any faster than three.
As a rule, you shouldn't build new nanoturrets if your current expenditure already exceeds your income. Each nanoturret spends about 10 metal per second at full speed, so you can easily determine how many you need.
DO have fun, respect your teammates, and enjoy all that the game has to offer.