School systems and schools do not all calculate weighted GPAs the same way. In fact, they can do it in ways that are wildly different. Weighted GPA can be useful for comparing an applicant to other applicants within a single school--to distinguish between the student who gets A's in honors and AP classes and the one who gets A's in standard classes--but I don't think it's good for much else.
If your school calculates weighted GPA, it's worth asking your guidance counselor what yours is, and if you're interested in applying to highly selective colleges, also what your weighted class rank is. That person will be your best source of information that's particular to your school.
If your school doesn't calculate weighted GPA, there isn't a whole lot of point in having somebody outside the school do it for you. Because schools' systems for weighting differ so vastly, comparing weighted GPA between schools isn't usually very meaningful.