Question: what coolant do I have? (or "what coolant should I use?")
Answer:G11 is blue (or bluish green). It's what came with most cars pre-1995, in about 1996 they started using the pink stuff (G12, below). G11 can be mixed with green stuff BUT it is not recommended at all. If it's blue in your expansion tank, stick with G11 and nothing else, just to be safe. Replace it every 2 years maximum.
G12 is pink. It cannot be mixed with any other coolant (except G12+ which is purple). It is supposed to be able to last 5 years before being changed (more frequent changes are suggested by vortexers, like every 2 years), and it has sealant and anti-coerosive additives.
G12+ is purple. It's new and it can be mixed in to pink coolant systems, it replaces G12 according to VW. In all other respects it is like G12.
Dexcool is orange. It cannot be mixed with any other coolant. It is very similar to G12 (though they cannot be mixed) in that it has a longer suggested time between changes (5 years) and has sealant and anti-coerosive additives. It's what I use, works great for me, others have had serious problems with it. Do not mix it with anything, stick with VW G11 or G12+ if you have the extra $20, probably worth it in the end.
Prestone is green. Most coolants are green, they are a-okay for most cars, but VW is not most other cars! This stuff is fine in VWs if all other coolants have been completely drained out, but why go to the trouble of removing a superior coolant and replacing it with something I wouldn't trust in my girlfriend's corolla? Use it if you must, but as a rule you should replace your original coolant with the same type, and for VWs that is not green.
*The G11 and G12 and G12+ coolants are all made by Petosin, a european company. If you really want to save a couple bucks you could buy petosin brand coolants instead, they go by the same color scheme (replace blue with blue, etc). VW sells it by the gallon, pentosin is sold by the 1.5 liter.
1 gallon = 3.79 liters
1 liter = .26 gallons
As for the additives, most parts stores (Kragen, autozone, pep boys, etc) will have them, use only those intended for aluminum heads. If you're looking to seal a leak, I suggest the stuff that comes dry with metal flakes in it. Get the prestone type if they got it, Bar's and others are also fine. Some suggest that a second dose might be necessary, I say it can't hurt. If you're looking to keep the temp down and prevent coerosion, then Water Wetter (by Redline) is supposed to be pretty good, but it's not necessary and in most cases it's not all that helpful (just more stuff that doesn't really help cool as well as just water can). What ratio to use:
VW recommends adjusting your mixture to your environment, if you need protection down to around -20 degrees F, then go with 60% H2O, 40% anti-freeze. If you need added anti-freeze protection (down to -35 degrees F) then go with 50% of water and anti-freeze.