Gameplay
The major methods for making money are scavenging, taking missions, and trading. Though any of these methods will yield sufficient credits to keep your ship upgraded, trading is by far the most lucrative.
Different space stations produce different items based on the facilities that the station has installed, and the resources available. A mining station will output a steady stream of refined ores. A barracks will produce more weapons and ships, manufacturing stations will make consumer goods, and commercial stations will tend to have a little bit of everything. Also, some stations are bigger than others, and will produce a wider variety of goods.
Stations will pay the most money for items that they don't produce. Mining stations will often pay well for consumer goods, for instance. However, certain items, like common unrefined ores, are traded openly. All stations buy and sell these items, and the price will never fluctuate, but different stations have wildly varying base prices. Once you have a rough idea of the prices for these goods, it's easy to set up a consistently profitable trade route.
One thing to keep in mind, don’t oversaturate the market. Consistently selling the same item to a station will drive down the price, cutting into your profits. Vary your trade route, and you'll see a consistent influx of credits.