You need to go to My eBay > Account > Subscriptions and choose which store package you wish to subscribe to.
Once you've chosen which store level to subscribe to, click on 'Subscribe', the next screen may tell you that the requirements to open a store have not been met - that usually means you have to put a credit or debit card (eg Visa, Mastercard) on file with eBay, as it acts as an account verification. There will be a link to register as a seller with a valid card on file.
After you do that, it's a relatively intuitive process to go through the steps and set up a basic store, but if you need any more help, just post the question(s) here and I'm sure someone will be able to help.
