Dakka has a
great guide on acrylic mediums that will give you a good basic idea of how things work. It is good to have the selection of matte medium, flow aid, drying retarder, thinners and distilled water. As Azreal said, I only use thinenrs for airbrushing. When brushing, I use water. Depending on the effect I want and the paint in question, additives and quantity of thinning varies greatly.
The best advice would be to get them all, read the article I linked, and practice using them on some test minis that you can strip and repaint many times. I use
AoBR marines for this purpose, they have bathed in nurgles rot many times, only to be born anew in Tzeenthchs fires of change.