Hello all. Quick tip for beginners here. This is my opinion though, so feel free to ignore it... :D
As the title says, always thin your paints before applying them to the model. Two or three thin coats are infinitely better than a single globulous one... (New word for you there).
I like to add two parts water to one part paint, but that's just personal preference. Always best to be safe though. If the mixture is too dilute, you can always leave it a few minutes and test it again until you're happy with the consistency. You're looking to avoid obscuring any of the detail on the model and the paint should flow nicely onto it.
You can practice on a sprue (or a metal model which you can then strip afterwards anyway) to get a feel for it.
This applies ESPECIALLY to white, which is, truth be told, a horrible shade to paint. The best way that I have found is to, starting from a Chaos Black undercoat, apply layers of grey, gradually getting lighter. So start with Codex Grey, then Fortress Grey, than a mix of Fortress Grey+Skull White then finally Skull White itself. Always make sure you have a uniform coat of the underlying shade before you add the next layer. This will help the white be solid afterwards. If it is splotchy, then just wait for it to dry and add another layer of the same colour until you have the smooth finish.
As long as you keep your layers watered down, the lovely detail on your model will remain intact, and you'll have a nice solid colour to bring that detail out.
I used Shadow Grey on the sergeant in this picture, but it's the same principle (he was (and still is, to my eternal shame) PIP at the time):