Step 1: Loosen and Verify Seat Post is not stuck. Adjust seat post up or down for test ride.
Step 2: Do the same with the handlebar stem. After all, stuck stems are a PITA, and at the very least, get a discount on the bike if it is stuck.
Step 3: Spin the wheels, make sure brakes work and are not rubbing, shift the gears. Inspect the frame for rust, dents or other defects.
Step 4: Inflate tires.
Step 5: Off for a ride!
Step 6: CRASH!!!!
Step 7: Pick yourself off the ground, knock the blacktop grit off your bloody hands, walk the bike back, and pay the seller.
Forgot critical step 2B: Retighten stem FULLY!!! Make sure bars are secure. It turns out, when you try to turn with a loose stem, the wheel goes one way, and you will go another....
Also critical step 4B: Put on your riding gear, at least the basics (helmet and gloves).