Early Game play
First thing you should do always is to capture the power right next to your command center (preferably a nuclear plant). It is usually the first income source. After that you have two options.
Option A: Make a barracks, this will allow you to grow militarily, and may be able to bring your little kingdom power that no one can compete with.(not recommended)
Option B: Build another nuclear Plant or a Power plant, this way you would have a lot more economic standing in the game. Allowing you to hopefully do better later in game, as money powers everything.(highly recommended)
Between Early/Mid Game play (if someone can give me the name of this stage please do so.)
Option A: After making the barracks, capture the nodes nearby and reinforce them with troops should the opponents are aggressive. Then, after establishing a good enough economy start building up an army that will be able to hold off the others and then destroy them.
Option B: After capturing the surrounding nodes and establishing an economy, make a tank factory or airport and build up an army, hold off the others if their attacking and then destroy your opponents.
You can make a research center to research Juggernauts then, make a fort to make them.(juggernauts are tougher than heavy soldiers but take up 2 housing spaces)
If you have made it this far into the game then you are skilled enough to do this.
Option A: sell your airport and replace it with a space link and start making space fighters.(note: motherships are expensive and are basically movable space links but are not recommended to be made)
Option B: Build a research center and research nukes help and aid you in destroying huge armies or obliterating bases