By design the web is normally open to everyone, and any websites you find on the web usually have free content. In some cases, however, you may want to keep premium content reserved for your paying customers or members. This members-only section may include pictures, videos, downloadable files, software or even just exclusive articles or books. To keep them exclusive, you will need to use some sort of paywall system.
Paywalls have received a bad reputation lately because some newspapers and magazines have started using them for their previously free online content. You can still avoid that stigma by making sure the content you offer as premium or exclusive is truly something people cannot get elsewhere for free.
How you setup your members-only area depends largely on how you deploy your website. If you use a content management system, there may be plugins you can use to setup a subscription system. If you have developed your own web applications, you will need also create the paywall system for it. Furthermore, you should also prepare your website to use SSL encryption to keep user data, such as email and personal information, safe.
The last thing you will need to consider is how you will receive payments and the types of payments you will accept. Many of those members-only plugins for content management systems support popular payment processors and integrate them well, ensuring the subscription process is automated. This will minimize the amount of extra work that might be involved with offering a subscription service.