It's been a while since the last time I played it, but let's see, if I can remember enough to help.

The expenses of a planet come from three areas:

1. Production (Military, Social, and Research)
2. Maintenance
3. Survival

There is not much you can do about the first point, except for lowering the overall production capacity you use or stopping to build anything on your planets (which still leaves the Research-cost though). The amount of production (and its cost) depends on the base value plus any bonuses from planetary improvements, starbases, and racial abilities (the bonuses are all simply added together). This value is then adjusted based on how much of your production capacity you are using, and on how much of it is used for the respective category.
For example, your planet produces a base of 100 industrial units per turn, and has 50% manufacturing bonus coming planetary improvements, a 30% bonus from starbases, and a 20% bonus from your Social Production ability. This means, that the planet can produce 200IU per turn when building improvements (100 x (1 + 0.5 + 0.3 + 0.2)). Your overall production capacity is set to 100%, and your Social Production slider to 33%. This means, that the actual amount of social production your planet will do is 66IU (200 x 1 x 0.33).
Each point of production costs 1bc, but a certain amount of your bonus production is free. I'm not sure, but I think, that it is 50% of the bonus production from starbases, 33% of the manufacturing bonus from planetary improvements, and 66% of the research bonus from planetary improvements. However, I could be wrong about this. I also don't remember, if some part of the bonus production from racial abilities is free or not.

Several planetary improvements will cost you a certain amount of bc for maintenance each turn. At the start of the game, you don't make much money, so you should be careful about what you build. The same goes for your ships and starbases too. Maintenance can quickly eat up your income.

Survival refers to the cost of colonising and maintaining low-quality planets. If I remember it correctly, planets with a PQ lower than 15 have a maintenance cost of 5bc. Planets with a PQ lower than 10 add an additional cost of 10X your government level (Imperial is level 1, Republic is level 2, Democracy 3, and Federation is 4). To give you an example, at the start of the game you're running an Imperial government, so a planet with a PQ of 9 would cost 15bc per turn to maintain.

Overall, the game is about colonising the best planets (PQ 15 and higher). Seriously, the PQ affects every aspect of your planets. For example, your income, production, and free production are being penalised, if they go above certain values times the PQ of the planet. However, I can't remember what those values were again.

In essence, unless you don't have any other choice, don't waste your time with low-PQ worlds. You can unlock a few improvements that can improve the PQ of your worlds (you already start with Soil Enhancement), but they only go so far (a combined bonus of 40%, if I remember correctly).

Well, I hope this helps.

Correct.

It's rare, that you'll be able to make full use of something. You'd have to set your sliders to 100% in one production area to achieve that, making you unable to use the others. Though, basically yes. If you build a factory on a planet you only want to use for research then that maintenance is definitely wasted. Though there is constantly waste in this game.

If you mouse over a post some buttons appear in the lower righthand corner of it. The second one to the right is the Quote button (looks like a single quotation-mark). Clicking on it will quote the whole post. If you only want to quote a specific portion of it, mark that part before clicking Quote.

Yes.

You can tailormake each world, but you're not able to make full use of it all the time. GalCiv 2 is slightly better in that regard, especially due to the Focus-mechanic (it allows you to use part of your expenses in two production areas to boost the third one, albeit with a penalty), but GalCiv 3 seems to have finally nailed it (based on what the devs have shown us at least).

Not necessarily. Sometimes it is necessary to run a deficit to achieve a certain goal, but you should still strive to have positive income (just like in real life). Depending on the approval rating of your colonies, you probably could have run a higher tax rate though.

You need to upload them to a hosting service first (like Dropbox or Imageshack), and then link to them in your post.

I'd first need to know what difficulty you played last.

>facepalm< Sorry. Missed that completely. I blame it on sleep-deprivation.

Going from Simple to Challenging is a pretty big jump, but it could work in a one-on-one. Even if it doesn't, you'll at least learn what you're up against, and can do better next time.