I started with a student credit card, and used it once or twice a month, and paid it off in full. Then I got a United MileagePlus card and I’ve been using it as my primary spending mechanism ever since, also paid off in full each month. I recently looked at my credit score when I bought my new car, and bam, things looked very good. So I think I am nearing completion of “building my credit”. I guess the responsibility over time pays off!