Hehe, it varies depending on what you want to do I guess. I use water and Matte Medium from Vallejo. I've seen Mix Medium from P3 (Privateer Press' range) at my local store and want to pick some of that up to try. There is an entire market out there dedicated to additives and mixes to put with paints, inks and washes.
As a rule of thumb, acrylic paint is mixed with water so that it flows on the palette. You want the paint to be a liquid, not a paste, but if you can spread it over the palette and it draws back into itself then it's too watery. For washes, GW stuff can usually be used straight from the pot (not recommended to have your brush quite that full though, so draw it from the lid), but you want paints that you turn into a wash to be of milk-like consistency. This is something like 10:1 water to paint ratio, but you won't get it exact unless you use dropper bottles.
Glazes are different, my glazes are about 4:2:1 water, medium and paint, and they're not supposed to sink into all of the depths but to instead tint the colour they go over into something slightly different.
This sepia is taking freaking forever to dry, so I'm assembling what I hope will be my next 2 models whilst I wait.