I find that most quite a few Tamiya Acrylics do not need to be thinned. A good rule of thumb is that it should be the consistency of milk. When I do need to thin it a little, I use straight tap water. I have read of many people using Windex, distilled water, and various other agents. If you want to find out a little more, use the search function and type in paint thinning, you should get quite a few threads to look at.
Happy Modelling and God Bless
Robert