Most high schools require 24 credits over 4 years to graduate. A credit is a year long class. So you will be taking 48 half-credits. This is important so you can calculate that each semester you take 6 half-credits (let's call them "units").
If you have a 3.25 and 6 units, you have 6 x 3.25 = 19.5 points.
If you get a 4.0 x 42 (units left to go) = 168 points
168 + 19.5 = 187.5 total points
187.5 points divided by 48 units = 187.5 / 48 = 3.91 GPA
If you take more or less courses, perhaps you can plug in your numbers and adjust your points accordingly. Better yet, head over to a website like www.afouro.com which has a pretty great GPA calculator where you can save your grades.