1.) gg and dG
The easiest way is to type "gg" to bring yourself to the first line of the file and the "dG" clears the contents.
2.) :1,$d
Hit Escape and colon and then type: 1,$d and all contents / lines of the file will be cleared.
vi, delete, gg, dg, easiest, quot, clears, contents, colon,
