Everytime I use my airbrush the paint seems to be too thin........and ends up splattering everywhere........can someone help me in how to mix the paint for it?

The standard guide is for the paint to have the consistency of whole milk. Another thing you may try is lowering the air pressure, and keep that brush moving constantly. Go for several light coats and build up the paint slowly to the desired opacity. It's also a matter of practice with your brand of paint, your compressor, and your brush together.

To add to what Greg said, you may also be holding the AB to close to the model.

Grifter and Twisted Fate hit the nail... Just follow their suggestions and things shall be better.
Ciao

To add to the others good advise--if the problem started after you have been painting a while you may have to clean out any paint build-up in the airbrush tip. Most of the time a quick swipe of a cotton swab (Q-Tip) does the trick.

If it is runny from the get go, then add more paint. The whole milk analogy is right on the money. I actually dip a paintbrush in the paint cup and then run the bristles on the lip of the cup and watch the speed and consistency as the excess paint runs down the inside. It is pretty much trial and error