Setting up and Apple Developer account is essential if you plan to distribute your app (s) on the Appstore. The process is quite straight forward. The first step is to follow the link on this page to the iOS registration page and then click the enrollment link
As displayed in the photo the cost is $99 yearly.
Once you click the enroll button its a simple matter of following the steps on the screen and working through the process.
If your registering as a company you will need to have your DUNS number
The DUNS Number is a unique nine-digit identification sequence which provides unique identifiers of single business entities, while linking corporate family structures together.
The D&B DUNS Numbering system is a widely recognised standard for identifying businesses among thousands of companies, marketplaces and regulatory entities. Companies worldwide use the DUNS Number to link information about suppliers, customers and trading partners, providing a more complete picture of the risk and opportunity in their business relationships. – More info
Once your application is approved by Apple you can then distribute free apps from your account. In order to sell apps and collect revenue you will need to complete your contracts, tax and banking information. Log into your iTunes Connect account with your Apple ID and password and click on the link – iTunes Connect