There are some websites where you can find a list of words starting with M, they are Dictionary.com, Collinsdictionay.com, Learnersdictionary.com and some other dictionary websites. Moreover, several websites like Wordbyletter.com or Crosswordsolver.com also offer such list.
The lists offered by these websites differ in the amount of words and the way they are presented. The first two websites allow you to browse the whole dictionary from A to Z. There is a huge amount of words in the lists and words are sorted by groups alphabetically for ease of browsing. Learnersdictionary.com, however, offers only a limited number of core words, also listed from A to Z, while Wordbyletter.com categorizes the words by the number of letters.