Don't stress... the newbie guide they link to should be helpful. If you spend even a few hours learning the basics, you'll be just fine. Don't try to do everything or you'll quickly feel overwhelmed... find what's right for you. :)
This is my newer merch business account, but you can also give my alter-go (@thatcryptodave) a ring. I still feel like a newbie though, so speak very slowly and keep the questions simple. 🙄 !LOL