'write what you know' is still the best advice for beginning writers... 'westerns' are, first of all, historical works about a certain type of folks who lived a century and more ago... so, you'll have to be something of a historian, to acquaint yourself with the life and times of the era, before even beginning to write about it...
you should also take jman's advice to read lots of the best westerns... bret harte was another prolific, award-winning western author... for a contemporary pov, we have tony hillerman's wonderfully-written views of life and sudden death among the navaho and hopi, in the great southwest...
for more info, go here:
http://www.westernwriters.org/