I can't read the postmark, but it is sent with a penny stamp.
As follows, exactly as he wrote it, misspellings and all:

Submitted by Jo Watts [email protected]

Addressed to Miss Annie Robertson (his sister)
Fitzhugh, Okla, box 5..... next nuimber is smeared

"Miss Annie Robertson
dear Sister and all I am leaving on the northern Picfic I am going to Camp dick new Jersey will rite as soon as I get there  you all be good and dont study aabout me for I will mak it all OK
I am having a joyful time with the girls at every Stop tell every body I sed hello and good bye

W.C. Robertson"