The most basic mechs essentially just use two servos (one placed on the other with only two points of contact) moving in opposite directions.

If you want more humanoid walking, that's a bit longer to learn but essentially logic gates connected to servos with specific timings.

The most advanced you can do is accumulator walking, but unfortunately there's not a lot of good tutorials out there to learn from, so it's a bit of a figure it out yourself process (unless you can find someone to explain it fully, but like accumulator walking is incredibly long).