USED wheels can be problematic for a beginner.
FIRST- Examine the rims to make sure they aren't bent from running into curbs etc.
Use an adjustable wrench for a "gauge" and run the jaws around the circumference to make sure the brake tracks aren't "splayed out.
That'll eat you up if trying to true from one side at a time. You get one side "true" and the opposite side is 2X as bad. You go to that side and make the problem worse.
One method (messy) is to take a felt tip marker. Spin the wheel and use the marker on both sides to just touch the high spots.
Examine WHERE the high spots are. If across from each other, the rim is "splayed".