There is a method in Autocad that make you able to use keyboard's letter as field of most useful commands in order to be fast, explain how to make that ?

By Creating shortcuts for commands, or in other words (Command Aliases).

Auto-CAD comes with a large number of aliases already made for you. You edit the acad.pgp file to add to these, or change them.

By editing "acad.pgp" file, which can be found in AutoCAD under "Tools --> Customize  --> Edit Program Parameter"