You can enroll one of two ways:

1) Online at: http://www.collegeboard.com/student/testing/sat/reg.html
2) By mail - instructions are here: http://www.collegeboard.com/student/testing/sat/regMail.html

Since you're enrolling for June, I would register online because the late registration deadline date is May 15, 2008 (source: http://www.collegeboard.com/student/testing/sat/calenfees.html). This way, you register as quickly as possible.

yeah, register online because it's fast

I just registered online for this exam last week. It's easy. Make sure you register for a Collegeboard first.