Things to do (or not do) in a game
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.
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.
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).
They'll just die one by one. You want to concentrate your forces wherever possible; see Lanchester's laws for why.
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.
Nanoturrets 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.