You know how you're always asked at the checkout counter: "would you like to apply for our fill-in-the-blank Store Card? You can save $X dollars on your purchase today..."
Don't do it. These pitches – a common part of the retail experience – are a potential credit pitfall. Applying for these store cards generates a "hard" inquiry that goes on your record, and is visible to lenders looking at your report. Every time you seek credit by applying for a credit card, store card, or loan – you generate a hard inquiry. Too many inquiries will flag you as a potential credit risk because it signals credit desperation. You should keep these to a minimum.
There are exceptions, of course. If you are shopping for a loan or a mortgage, a lender will expect to see a short burst of inquiries against your credit score. It's best if these happen fairly quickly and around the time of a loan event.
There's also such a thing as a "soft" inquiry; only you can see these, and they do not impact your score. Potential employers might make an inquiry, for example. And when you check your own credit report, your inquiry is both invisible and irrelevant to your credit score.
Make a habit of checking your credit score each year – and watch how those good credit habits push your credit score skywards.