Country Music Hall of Famers who did/do not write their material (or wrote very little):
Tex Ritter
Red Foley
Gene Autry
Patsy Cline
Kitty Wells
Vernon Dalhart
Little Jimmy Dickens
Tennessee Ernie Ford
Patsy Montana
Brenda Lee
Elvis Presley
Charley Pride
Carl Smith
Glen Campbell
DeFord Bailey (Bailey didn't record much, so this isn't a fair assessment)
George Strait
Sonny James
Emmylou Harris
Barbara Mandrell
Charlie McCoy
Roy Clark
Ferlin Husky
Don Williams
On the other hand, these people are in primarily as songwriters:
Fred Rose
Felice & Boudleaux Bryant
Cindy Walker
Harlan Howard
(Billy Sherrill is also a songwriter, but his bigger contribution to country music is in record production.)
There is no way to tell you how to write songs. To be successful you have to write from the heart. Formulaic material is obvious and it wears thin quickly.
If your goal is to "be big," forget country music and go into acting. Music takes far too much dedication and devotion, and 99% of people who "want" to be famous never EVER get heard outside of the bar they play in on the weekends. You need to work hard to develop a following where you live, because nobody from Nashville is going to be interested if you're not good enough to get your neighbors interested.
You have to develop YOUR sound. Don't try to sound like whoever is popular right now, for two reasons. First, there are a million other people impersonating whoever is popular right now, so you won't stand out. Second, in the time it takes an act to get heard, discovered, signed, recorded, and released whoever they're impersonating can go from #1 act to the "where are they now" section on "Entertainment Tonight."
Most importantly, if you're going to call yourself a country singer ***SING COUNTRY MUSIC***. Do NOT sing pop or southern rock and call it "country," because you're lying to yourself and your listeners or potential listeners. You should know the history of country music so that if someone comes up to you and asks for "I'm So Lonesome I Could Cry" you don't embarrass yourself by saying, "Who's that by?"
Finally, and if you pay attention to nothing else, read and heed this: before you do ANYTHING, read and study about the music industry. That might be enough to cure you of wanting to be a "pro singer."