For university assignments, refer to your lecturer or tutor as they style to choose often depends on the subject you are studying and the preferences of the academic staff.
If you are submitting a manuscript for a journal, refer to the author guidelines provided by the journal you intend to submit to.
It is possible to create a bibliography in Word using Mendeley Web. For details see this Mendeley Support page.
Note that "MS Word’s citation system is not the same system used by Mendeley. Using both on the same document will yield two sets of citations and two bibliographies".
If you are using Office 2016 downloaded from UCL, you may find that you are unable to install the Mendeley Cite-O-Matic or Mendeley Cite plugins. UCL has a policy which prevents third party Office add-ons. In such an instance you may get the message 'We can't load this add-in because we couldn't connect to the catalog'.
There are some alternative options for working with your Mendeley library in Word: