Need Serious Economic Advice

It's hard to give specific advise without knowing more about your empire. How many planets do you have? Are they specialised or more general? What is the max population of your planets?

Still, here is some general advise:

1. You can't earn taxes without a tax-base, so get a high population on your money-making planets. 15b is the minimum. If the planet has a PQ of 20+ then go for 20b. However, never go beyond that. A population of higher than 20b is just too difficult to keep happy.
2. Increasing your population also has two more benefits: it increases your influence, and it increases the total amount of tourism income in the galaxy. Your share of the tourism income depends on how much of the galactic influence you produce. For example, if you produce 20% of the galactic influence, you get 20% of the tourism income. For this reason, researching the culture techs (or anything that provides an influence bonus) can be quite beneficial.
3. Research the advanced governments, if you have them in your tech tree. Interstellar Republic, Star Democracy, and Star Federation increase your economy by 10%, 20%, and 40% respectively. So get them as soon as possible.
4. Specialise your planets. This way you can better maximise your bonuses to manufacturing, research, and economy.
5. Keep an eye on maintenance. There are several improvements which are supposed to be upgrades, but are actually not worth it to use them. For example, the Industrial Sector factory has a cost of 400, a maintenance of 10bc, and provides 12mp. Its direct predecessor, the Manufacturing Center, however, has a cost of 150, a maintenance of 6bc, and provides 10mp. So you effectively more than doubled the cost and increased the maintenance by 2/3 just for 2mp more. You're better off staying with the Manufacturing Center, unless you have a planet with lots of manufacturing bonuses to make good use of the higher base production.

Once your treasury goes into the negative, your approval rating will slowly begin to drop, and you are no longer able to rush-buy things. Once your treasury reaches -500bc or lower, all production (manufacturing and research) will stop until the treasury is above -500bc again.

First of all, reduce your tax-rate until your approval rating is at 100%. Your population-growth gets doubled at 100% approval (even more if you have the Super Breeder ability). Try to stay at that level as long as you can. Your tax-base will be much higher that way later on.

Research anything that can boost your economy. The best techs are those that provide bonuses to economics, population-growth, and approval.

Building two or three factories on your homeworld is necessary to produce new colony ships as quickly as possible. However, try not to over-extent yourself. Each new colony costs 10bc.

Don't immediately start building improvements on new colonies. Wait until the population is high enough to pay for the colony. In some cases, it's necessary to begin building earlier. You have to look at the situation.

Don't spend all your money on rush-buying. Your starting money is your safety-pillow. Try to make it last as long as possible.

If you are playing as the Terrans or the Drath, and start out with other races close by, then it might be a good idea to go for Universal Translator ASAP. This way you can start tech trading much sooner. Due to your huge diplomacy bonus, you're bound to get good deals.

I probably forgot a couple things, but that should be enough to get you started.

Thanks a lot for taking the time to respond with so much info.  I posted this kinda late last night and realized after I did that I left out some important details.

I'm at work right now but I can give you an idea.  So i have about 19 or so planets (+/-1) and early on I made the mistake of specializing them but I focused almost entirely on industry and research.  I know I know...I'm still learning.  I started having money problems early on so I began building banking centers (or equivalent) here and there on planets.  I have two that are predominantly economic but they just don't generate that much money.

When the war started I didn't realize that troop transports were billions of people.  I thought it was 2,000 troops I was sending in, not 2,000 million troops.  I also didn't realize the population of the planet I was invading would be completely wiped out.  So I inadvertently gutted the pop on some of my planets and because the cost of production is so high, my taxes keep approval consistently at around 50%.  I haven't researched other governments yet so this hasn't been a problem from a rebellion standpoint.  I get now why morale is important though.

The bottom line is that I know I can win this war, and this game has been incredibly fun.  I really don't want to bail on it but losing this much money per turn is crushing me.  Do I drop taxes to nothing and production to nothing and just hunker down for a bit until my pop recovers?  The computer players have so much money it's insane and before I sold some tech I thought I was doing great with 5k in the bank.  LOL!  Now I realize how awful my economy really was this entire time.

I'm afraid that if I don't keep up the offensive my opponent will eventually overwhelm me because a war of attrition I cannot win unless my production is at full.

Establishing an economy is the most important thing in GalCiv, because without you won't be able to do anything. So if you want to do anything, always pre-plan of how you can pay for it.

Techbrokering is one method, but it can be very dangerous because you sell your headstart to possible enemies (who would use these techs against you). The game is easier when you leave the AI "stupid" (without techs)

Better is to have your income come from your planets through a high popultation, lots of economic structures and high morale (so you can set tax slider higher) Try to increase all 3 of them, first focus on population, then on tax & econ buildings. After a few games you'll get a feeling for it.... first and usually your economy will decline during the colony rush (this is actually the hardest phase in the game, because you don't really have any popultion here...) then it will rise slowly and at some point, you'll make money, every turn a little bit more. Then here's the time to re-concentrate to build more factories and labs, and not banks anymore.

The most easy way however is to play with Torians Super-Breeder and adjust sliders to have 100% morale continuously.