First thing first, start a tiny galaxy with a single ennemy set to ultra-dumb, disable all win victories except diplomacy.
From there on, test out things, take it slow, go back to the tutorials once in a while (you can access the main menu without losing your game - simply click on the "continue" option in the main menu.
There's a few basic concepts, outside of gameplay tips, to understand:
1- Your planets are your "cities" from Civ4. They produce things much the same way it would happen in CIv4. Differences are that you don't have to assign workers and that workable tiles are shown in the planet view. You can build things on those, which are like building improvements in Civ4 - but in GalCiv2 those are limited (obviously) so choose what you want to build
2- Colonies. To set up new colonies you need 2 things: A spaceport to produce a colony ship and a colonisable planet (means a planet with a class above 0) Build your colony ship then send it over to the planet (move it "in" the planet) and you'll then be able to colonise.
3- Taxes. Each production point (military, civic and research) cost 1 BC each to produce. Taxes come from your tax level (from the Economics window, accessed from the main menu at the bottom). The higher the tax, the more money, the unhappier the people. Spending level means how much of your industrial capacity you want to use. At a 100% it means you want your factories to run at a 100% efficiency. Obviously, the higher your spending level, the more you're going to spend.
In the same window, at the bottom, are the spending levels for each industry categories (Military, civcs and research) There dictate how much of your treasure is being spent on each sector. Rising one will send more money to it at the expense of the other 2 sectors.
4- star bases: One other ship type you can build are Constructors. With these, you build space stations. Space stations are great, there are 4 types:
- Mining starbases : You get them when you find a galactic ressource (those colored polygons parsed over the map) They give you bonuses. For example, Red is a military bonus, Yellow is a moral bonus.
- Military starbases : These are meant to give bonuses to your ships that are in their vicinity . Great for setup up "Fronts" agaisnt an ennemy.
-Economy starbases: These give production bonuses to your planets that are within their area of effect, can also give a bonus to trade lanes.
-Influence starbases : Incrase your borders with these things!
How to build a starbase should be covered in the tutorials.
That's about it.
You need to always understand the circle of money:
People pay tax, tax get spent on industry, industry builds things(space ships and buildings) and makes research.