What was William t sherman first major accomplishment and how did he achieve it?

Apart from the March to the Sea (Nov-Dec 1864), there was no big, single achievement.

He had performed well at First Bull Run - which his side lost.

His next achievement was to become a good team-partner to U.S. Grant, and helped him liberate the Mississippi. As a reward, Grant made him an army commander in Chattanooga, and told him to destroy the Army of Tennessee. It was actually his failure to achieve this that prompted the March to the Sea.

When that was over, he invaded South Carolina, then North Carolina, taking the surrender of Joseph E. Johnston.

Aside from battlefield operations, he was notable for being one of the few people at the beginning who judged that it would be a long war. And he wrote a classic book of memoirs, though he never looked like the sort of person who would write a good book.