Need help on what to do...

..the other day I was stopped at a light and the van just kind of stalled
out (while still in drive)...It wouldn't move when I pushed on the
accelerator (the engine still reved up like normal, the car just didn't
move... Previous to it finally going out, the rpms seemed to be really low
(like 400ish) and jumpy when I'd release the gas pedal while driving (felt
like the idle would drop quickly).

Once it wouldn't move the dashboard said "Check gauges"...I didn't notice
anything out of the ordinary besides the rpm's being on zero and the oil a
notch above "L"...In order to get back to my house I just put the van in
park and restarted the car and I was able to move again. I know I need an
oil change, but what exactly is the problem here and what do I need to do
(or who I need to see) to get this problem fixed...thanks in advance.