Anki and AnkiDroid use exactly the same file format, so you don't need to convert any files. In general, see
on moving cards between Anki and Anki Desktop.
However, if you've only been given a .anki2 collection file (I hope it's .anki2 not .anki because otherwise it would be from a really old version and need
some extra work), you can't just add them to your current collection. The easiest way is to "load" the collection in Anki Desktop (look for the current collection.anki2 file in your Anki folder, rename it so you can restore it afterwards and add the collection.anki2 file you've been given),
export it as an .apkg file and
import that on your phone (you can do the same on your phone if you prefer, but it might be easier to move the files on a computer).