Students should take the SAT and/or the ACT anytime their Junior year and through December of their Senior year of high school.  It's best to start in junior year because it allows time to prepare and retake if unsatisfied with the initial result.

Students should take the PSAT in October of their Sophomore and Junior year.  The Junior year test counts for the National Merit Scholar competition.  It's worth taking Sophomore year to get a practice in before the one that really counts Junior year.

It's probably best to take both because there are scholarships that require the SAT and others that require the ACT.  Additionally, some students perform better on the ACT compared to the SAT and vise versa.  

The SAT tends to ask more straight-forward math questions and tests more vocabulary than the ACT.  The ACT is basically a giant reading test: English includes grammar, punctuation and reading comprehension questions, Math is mostly word problems, Reading is reading, and Science is reading charts.  

Venmo is the preferred payment method.  Zelle or cash also work.