Standard ADM or JDM wheel?
If ADM:
get small screwdriver, pry the center "S" or "C" badge out from the steering wheel. Put the screwdriver underneath the badge in the middle and pry. Should come out easy. Then there's one 17 or 19mm nut (i forget) to undo. Undo it most of the way but don't remove it yet. Then wiggle and pull the steering wheel (can take some force) toward you to til it comes loose. THEN remove the nut and take the wheel off.
JDM: same as above, but instead of prying out the badge, you just pull the 'toyota' bit off towards you.
Fitting the aftermarket wheel should be very straightforward.