Make Up One's Mind Meaning

(idiomatic) To decide; to reach a conclusion.

Example:   I can't make up my mind whether to have ice cream or cake.‎
1909, Archibald Marshall, The Squire's Daughter, chapterII:
  "I was dragged up at the workhouse school till I was twelve. Then I ran away and sold papers in the streets, and anything else that I could pick up a few coppers by—except steal. I never did that. I always made up my mind I'd be a big man some day, and—I'm glad I didn't steal."