The easiest way to thin your paints is with good old-fashioned water. Acrylic paint is a water based medium, so you can simply add more water to this mixture to increase the fluidity of the mixture to the desired consistency. You can use tap water, which is by far your cheapest option, but additives and minerals in the water can alter the paint or affect drying time, so at the very least i would recommend filtering the tap water if at all possible. A cheap but effective option as well is distilled water. You can pick this up at any local grocery store or pharmacy and will only run you a dollar or two for a gallon which will last you for a long time. Now, acrylic paint as we said earlier is pigment suspended in a liquid, in water pigment tends to settle to the bottom of the mixture, and this will manifest itself if your mixture has too much water. While it is a great place to start with learning how to thin your paints, it is something to be aware of. I have heard of painters using Windex or other glass cleaners as well as glossy floor finishes mixed with water as thinning mediums, but they can be tricky and depending on the chemicals in the actual cleaner or polish will have variable drying time and will require some experimentation to get right.
There are also a number of commercial thinning mediums available as well, and most miniature companies who offer a paint line of any type often have one available, the only downside is that these can be quite pricey at times for not a lot of product. A bottle from Vallejo for example can run you anywhere from $5 USD to $20 USD for a small bottle of only 17 millilitres. Another option available is the use of extenders, otherwise known as retarders, that increase the working time of acrylic paints by extending the drying time while diluting the mixture. Professional art supply stores and even your local hobby shop can carry these from companies such as Liquitex, Winsor and Newton or Golden. These same companies also make airbrush flow aid mediums which can be used as a thinning agent as well. They can be quite pricey at times, but smaller bottles as well as items on sale and coupons can help you save some money. You are going to want to experiment to find what works best for you and your miniature painting.
While I'm on the topic, I would like to take a few moments to share with you the mixture I use on a daily basis. I picked up the recipe a while ago from another commission painter, and it is the best I've found in ensuring easy mixture and flow of paint as well as extending drying time while keeping the colors of your paints bright.
Here's the recipe for your enjoyment:
40% Drying Retarder
50% Distilled Water
10% Flow Aid / Airbrush medium
What I do is pick up a 10oz squeeze bottle from Sally's (or another, regional beauty supply store.) It comes pre marked in ounces, which makes it much easier to measure out. If you can't find a bottle with measurements on the side, that's quite alright. Get yourself a bottle of slow dri or another drying retarder, I perfer Liquitex Slow Dri which comes in a handy 4oz bottle, and a gallon of Distilled water from a grocery store or drug store. This small sized bottle of Slow Dri can be picked up at a local art supply store for under $10 USD. Throw the slow dri in the 10oz bottle, then if your destination container doesn't have measurements, fill the now empty bottle to almost the top with distilled water and then pour that into the 10oz bottle. Fill the rest of the 10oz bottle with flow aid or airbrush medium and you then have yourself a thinning medium that not only will last you a long time, but will increase your paint working time and improve your time as a miniature painter.
The benefit of this mixture is that it will thin your paint while helping to suspend the pigments, increase drying time and normally winds up being cheaper than most stand alone thinning mediums. it also makes mixing easier as mixing it in a 2 to 1 ratio of paint to medium comes out to the perfect consistency. I have painted a few hundred miniatures since the last time I made a batch of this and I'm only barely halfway through the bottle.
Whichever method you use, I highly recommend thinning your paints, even if they claim to be pre thinned or airbrush ready. Your miniatures will come out looking much better and you'll have a far easier time painting your miniatures.