Income in oGame is measured in Metal, Crystal, Deuterium and Energy. Generally you would increase these by building alone, but there are several other ways to gain advantage over other players.
1. Asteroid Fields
If you see a destroyed planet where the number has an A next to it, you can attack it and gain half the resources. While most players move their resources off it first, you can still gain small amounts. Always send an espionage probe first though, if the planet had defenses, they are still active.
An asteroid field is generally there for 24 hours from when a planet is destroyed. In some cases, players have reported it in place for much longer.
A little better additional source of income is recycling. To do this you need to build recyclers, so get that tech up there and get them. If you look at the galaxy pages, you will find that some of the planet numbers have a T by them.
Hover over it with your mouse and you can see how much Metal (M) and Crystal (K) are floating there. Send a recycler to the debris field of the planet to collect the resources. You can pick up as much as your recycler can carry up to 100% of the resources there. The owner of the planet does not see you coming, and there is no chance of losing your recycler, so don’t bother to send other ships as escort.
Recycling Made Easy – Your guide to Debris Field is a great way to learn how to send those recyclers.
This can double your income almost instantly. There is a completely separate topic on raiding, that you can find here.