As per Apple's guidelines, an Apple Developer Account is mandatory nowadays for e-commerce store owners. Here is a step-by-step guide to create the developer account with Apple.
Step 1: Visit the Apple Developer Account enrollment page by clicking this link: https://developer.apple.com/enroll/. Once you open the link, you will be asked to sign in to your Apple account. You can use your iCloud mail ID or the mail ID you have used to log into your iPhone, iPad or Macbook. You can also create one right there if you don't have either of these.
Step 2: Once you log in, you will be asked to choose your entity type. An entity type would mostly be the Company/Organization (if you have a proper registered company). If you don't have a registered business or if you're running the Sole Proprietorship business you can choose the first option - Individual / Sole Proprietor / Single Person Business.
Choose your applicable entity type and then click Continue.
Step 3: If you have chosen to create a Company type account (i.e. if you have selected the entity type Company / Organization), you will be asked to confirm whether you have the authority to create the Developer account on behalf of your company.
Step 4: Next, in the same page, you'll be asked to fill your contact information for verification purposes. Take measure to fill in the details properly. Ensure that the first name and last name you fill here is the same as the one entered for your Apple ID with which you logged in, in Step 1. In some instances, Apple may want the Apple ID information and verification information to be the same.
Step 5: Now, fill in your Organization information. The legal entity name would be your registered business name. Next you need fill in your D-U-N-S number. Apple will request it to verify your business' identity.
DUNS number is something assigned by D&B to all the businesses around the world. If you don't know your DUNS number or if you haven't got one yet, you can get it free by filling in your company details in this link: https://developer.apple.com/enroll/duns-lookup/#/search.
Note: If you want to bypass the DUNS number just sign up as an Individual in Step 2. You just need to remember that your name will appear as the developer in the iTunes store.
Step 6: In the next step, you will be asked to confirm the accuracy of the details you have provided thus far. Verify your details, then click the Continue button.
Once you click the button, you'll be taken to the final step where you need to complete your purchase by making the payment. An Apple Developer account will cost $99/yr. You can make the purchase online by clicking the Purchase button.
Step 7: Once your payment is complete, within 48 hours you will receive an e-mail from Apple requesting you to activate your account. Click the link in the email to activate it (the activation code present in the e-mail is clickable).
Once you click the activation link, your developer account will be activated. Voila! Your Apple Developer account has been created!
To visit your Member Center, go to: https://developer.apple.com/account/. There you will find the option to invite your developer so that they can log in, create certificates and upload your iOS app on your behalf.