You make money either through selling food on the market, or through plundering planets.
If you choose to plunder, just build pure housing infrastructure on your planet, and wait until you have max pop (the money you get from plundering is based on the planet`s population). Ensure you have some way of getting food by building one or two clusters to provide it for you. Research housing.
As for agriculture, you make more money, but you need more clusters. Build just enough housing to fill up your planets. Then fill up the rest with agriculture. Forget industry and commercial. Research pure agriculture. Sell your food on the market (I suggest 1 credit lower than the current price).