What is a membership site?
A membership site provides information or services to those users who register and pay to become a member. Some membership sites charge their members on periodic basis, while other have a one time fee. Once someone signs up as a member, depending on the owner's configuration, they are automatically billed each month. Those who sign up to become members have access to member's only information, benefits or discounts provided by the site owner on a regular basis.

Information Products
If you have material about a certain subject there are a couple of ways you can go about starting a members only website and making a profit from that knowledge. You can start a blog type of website where visitors can signup, for a fee, to receive more valuable information updates from your website.

You can also set up an information based site where members have a set of tools to collaborate with other members around the chosen subject. This reinforces the sense of community which increases the retention.

You don't have to be a constant presence either. Simply let your members know that you will be available X amount of times monthly or weekly from say 12pm to 2 pm to answer questions... you get the idea. Another option that won't require a set time commitment of you is to create a forum where you can control your schedule to help members as you can.

If you are not an expert but intend to learn about a specific subject, refine a certain skill or even study how to play the guitar, you can become advocate for people who have the same interest. Call a couple of experts in your area of interest, record presentation and make it available in your website.

Member's Only Bonuses
Offer more information of your services to members at a special fee. For instance, if you have a fellow business owner who sells a product your members would find useful, ask if you can set up a special deal for your members only. Most business owners will agree since there's minimal investment for them.

Do you offer a service or know someone who does? Again offer your members a discount created specially for them.

Affiliate Programs
Affiliate marketing is another way to generate sales from membership websites. If you promote products as an affiliate, be sure to include those promotions in emails and posts to your members. As with the previous money-making option, you might even be able to set up a discount just for being a member or give them an added bonus of your own when they purchase through your affiliate url.

If decide to send affiliate offers to your website, make sure the discounts you're offering is related to your main subject and important to its members.

Once you have decided which direction you want to take, you need the right tools. There is a lot of information available about how to start a membership site all the way to upscale hosted platforms. Sites like iLearn Online Marketing or My Membership Empire have vast training courses, membership site blueprints and materials.