For membership in mainstream it is almost always a service you want to offer.
First of all you really wanna reconsider who you want to offer this service for...
The private consumer is very unlikely to signup for something that costs money. It is incredibly hard and is not what I would recommend you to start with.
Start out with thinking of something you can offer small companies that could hold significant value for them.
Could be a online task management system for only $99/month.
The single most important thing is that your product has some real value! Otherwise you are biting the dust before you even get started.
Mainstream subscriptions is a whole other ballgame where your focus have to be on small business rather than private persons. Think about it, what do you subscribe to that costs money? NOTHING is the correct answer
Target the Fortune 5.000.000 of businesses and you are off in the right direction....
Best of luck.