Graphics calculators are a gimmicky toy invented by calculator companies to rip of high school kiddies.
No mathematician, scientist or engineer ever uses one.
Consequently, unless you need this thing for your high school course, I wouldn't bother buying it, and I wouldn't bother wasting time trying to learn how to use it.
BTW, I was looking at the ACT senior highs school maths syllabus the other day, and I think they do use them in the ACT.
As for sites about how to use them, why don't you try CASIO and TI themselves.