It all depends on your budget, and how much time you are prepared to put in practising. First and foremost, I recommend you find a paint that is available easily, has a wide range of colours, and doesn't cost an arm and a leg! Once you find it, stick with it, it's the only way you'll master it. As for thinning it, water is as good as anything if brushing, or use a good acrylic medium, which helps keep the viscosity even. Have a look at Grumpa's builds, he uses craft paints that can be found in every town:
http://www.scale-models.co.uk/threads/jims-way-dry-brushing-and-weathering.27390/#post-354845
There is always a large rack of them, and they're around £1.20 uk. I've used them too, and once you are used to them, they are excellent. As you will see from Grumpa's builds, they certainly don't restrict what you can do, but you do need to use them in thin layers! This can be disheartening for the beginner, as your model will look terrible for the first few coats, but keep at it and you'll soon have a surface that is complex and deep, and therefore, far more realistic.
You can also use Vallejo for brush painting, and the
Revell and Humbrol acrylics, but you'll spend a lot more money on them for a lot less paint, so if you are on a budget, have a go with these - after Grumpa posted about his method, I had a go on an old T62 that hadn't come out as well as I hoped, I was amazed at the results: