You can sell minerals, food, ore, maybe consumer goods and raw materials, perhaps even ask players for help with artifacts.
If there is truly no way for you to get out of debt, you should restart your empire.
If you`re Terran, focus on building commercial infrastructure on your planets and either build the same amount of agriculture as housing or buy the food from the market. Try not to build ships till you have a couple million credits, as the upkeep can go quite high, even with scouts.
The general rule of thumb is to research commercial to lvl 20 before anything else, then research all ships and technology except kalzul overmind project. After you`ve done that, you should try exploring for planets to increase your income further, or go straight into fighting for your planets.