Thin paint may be harder to controll and needs to be applied in more than one layer most often than not - but it achieves much better results, doesn't create texture on the surface of models, clogs details and so on. Also, your burshes tend to last longer.
Maybe start by adding a drop of water to your paint and see how you can manage with that. Ideally, when you run your brush through the paint, ii shouldn't 'push' the paint forward, but the paint should 'flow around' it. Don't know how to describe it otherwise.
Important is, that you always 'unload' your brush before applying the thinned down paint. For that, tip the end of the brush (the thick part) sideways onto a piece of toiletpaper (or the like). This way, excess water and paint is soaked into the paper. Otherwise, all that watery paint will flow everywhere onto your miniature as soon as you put the brush on it. The water will drag all the piment with it into the gabs of the miniature and not much will stay where you actually want it.
Also, prime your miniatures. I'd suggest white or grey. Even better, spraycoat it in grey. Than spraycoat it white but just from above/slightly angled. This way, when you apply thinned down paint, you will already have different shades of colour, allowing you to see where highlights and shades could go - or even just apply one layer of paint and call it a day :P.