Being unsure of your credit situation, definitely build credit as soon as possible. Get a credit card that you autopay in full every month. A personal loan seems unlikely to be approved to me, but the interest rates would crush your margins anyway. To put it in perspective- you self fund and have success grow and build credit at the same time. Grow bigger than you can handle, then start hiring and keep going. B) you somehow get approved for a personal loan at 15%. First/ second payment due before your shirts are even delivered, how are you making those payments? Your sales slow and now you are working a job to pay off debt and have boxes of shirts in your parents garage. Be patient, good businesses take time.
My experience: I’ve owned my business for 9 years, we do well over a million dollars in revenue, are profitable and still struggle to get financing for big equipment that would increase our margin.

Before you even worry about this… no one is gonna loan you the money anyway (outside of family/friends.) You have no business plan, 1 sku that sold out 1 time in nominal quantities. I’m going to assume you are limited in assets since your 18. Go get a job and use the income to fund this if you really believe you can do it. 50 shirts, reinvest to 100 shirts… etc. Retail apparel at 18 is gonna be a bootstrap mission, let FUBU be your guide.