You an actually use water. Typically it's 1/2 cup of water per gallon of paint. But I would recommend stirring a little at a time until you get the consistency you want.
Yep, water will work. Add a little at a time, and stir thoroughly until you get the desired consistency. Start with 1/4-1/2 a cup per gallon.
I use warm water slowly as suggested above.
If you're ever in a pinch again and need to know what to thin out a product with, look for the cleaning instructions on the label. Usually latex paint says to clean up with soap and warm water. Obviously you wouldn't put soap in your paint to thin it, but warm water is great for thinning latex. Oil based paints usually say to clean up with mineral spirits or paint thinner. So that's what you can use to thin in out.