Make Up One's Mind Meaning

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."