Generally, it works kinda like this:

止まる【とまる】= To stop → 止まります【とまります】
Because this word is read tomaru (the sound before the る part is not an え sound) you can expect to be able change the る part to り and then add ます.

止める【とめる】 = To stop (something) → 止めます【とめます】
Because this word is read tomeru (the sound before the る part is an え sound) you can expect to be able to remove the る and add ます.