Many university registrar websites have instructions for calculating GPAs. Make an excel spread sheet that has the classes you have taken with the number of hours and the grades in separate columns. Then continue adding the courses you are thinking of taking later until you have a full 4 years schedule.
Use the advice above to use the Excel sum functions etc to calculate your GPA. First do it for the classes you have already taken, compare that to your transcript to make sure you are doing everything right. Fix the formula in Excel until you get the right GPA (what your school calculates).
Then you can have this document to add courses and grades to each semester and also to play around with by adding planned courses and hypothetical grades to see how your GPA will play out.
Edit: I think that it is a valuable use of your time to learn how GPAs are calculated and be able to create this document yourself. Nonetheless, here is one template I found by Googling "GPA calculator excel template"...there are a bunch of things out there
http://office.microsoft.com/en-us/t...e-tracker-and-gpa-calculator-TC001019773.aspx
Edit2: This one has a conversion chart for grades
www.sonoma.edu/is/abroad/gpacalculator.xls