ALSO
if a wheel is significantly bent (like a potato chip as
Andrew R Stewart shows above) then it is best to bend it back as best as possible before you start trying to true with spoke tension. Push the axle against a tree or something and push on either side of the rim to bend it back to close-to-straight.
A wheel in this state is basically ruined and needs replacement (the rim at least), but can be made safe to ride until replacement is possible.