If you are playing the original Gal Civ II or the Dark Avatar expansion, you'll start a standard sandbox game with 5,000 Galactic Credits (GCs). If you own Twilight Of The Arnor, you'll start with 3,000. Either way, you'll be starting the game with a significant amount of cash reserves. And while it may seem sensible to save the cash in case you need it at a later date, you actually need to spend, spend, spend.
The first thing you should do in every game is turn you overall production slider up to 100%, and lower your tax rates until you have 100% approval. You'll never need to touch the production slider again, but as the game moves forward, you'll need to lower your taxes constantly in order to maintain 100% approval. By doing this, you'll gain a population growth bonus that will pay off massively. More people means more people to tax, and thus more revenue overall.
Low taxes however, combined with the strain of new colonies (which cost more money to maintain than they generate until the population becomes larger) will cause your fledgling empire to plunge into an arterial spray of red ink.
But don't worry. Instead, get your flagship into the stars and claim every anomaly possible. With a little luck, you'll run into several anomalies which grant you a cash bonus of between 250 and 1000BC. These will keep your empire running. If you're playing on one of the larger galaxy maps, you should invest into sensor tech early. This will allow you to build more ships with survey modules, increasing the rate at which you can claim anomalies.