Ive used myairbrushes on and off for longer than I would like to admit, though I started out using a Badger 200 to paint my WWII aircraft models. I started out using mostly oil base paint. They coat and dry wel, but I switched toacrylics so I could avoid the vapors and using so much cleaning solvent. For the acrylic paints, I'd agree that milk is about the consistency that most experienced modelers recommend. For a beginner, I might not try it quite that thin as it will run pretty easily if you get greedy. Try to work in light, even passes until you get the coverage you want. if the mini looks really wet and you think it needs more paint, just pause and let it dry for a few moments before you add more. That will help minimize runny paint or overly heavy coats that fill up your panel lines, A good practicing tool is to spray onto the side of a milk jug. It's non-porous, so it will teach you to use lighter, even passes to prevent running. Using paper to practice is fine, but the plastic jug is less forgiving and will teach you more.
I have used the GW paints and I think they work pretty well. Straining them helps remove some of the clumping but they behave pretty well and dry fast. I mostly also use Vallejo Model Air. They have very fine pigment and are ready to use right from the bottle. They dont dry that fast so you have to watch out for runs though. In my experience, I dont find that Vallejo is very durable so be careful handling the minis until you dullcote them. I have also used Tamiya and it sprays well, though it has a bit more solvent in it. It dries and coats well, but since it is a hotter mix, it also has a little more vapors to it. As I mentioned in another post, I have experimented with straining some of the cheap Americana craft paint and it did pretty well in my first test.
Keep some Q tips handy as you can use them to GENTLY wipe off any paint that starts to accumulate on the spray tip. This is paint that drys a bit and clumps up at the tip. Once it builds up enough, it will blow off and splatter small clumps of paint onto your target. So the Q tip will help avoid that.