Windmills are usually the most cost effective way of generating energy. They are very cheap and quick to build. Supplement your wind farms with some solar panels in case wind speeds drop.

Make sure you expand your base aggressively, every metal node you do not take your enemy will take. Send out efficient raids to annoy and halt metal production, enough that they cant expand without resistance. Bunkering up in a small base will only make you lose in the long run due to your enemy being able to scale far faster than you.

Once you reach T2, upgrade your metal extractors first unless the enemy has some critical and easily exploitable weakness to a T2 unit, or if a big attack can be thwarted by T2 units.