There's only two types of membership:
a) Enterprise Program ($299) - If you represent an organization of 500 persons or more and want to distribute your application(s) internally (not via app store) and/or on the app store.
b) Standard Program ($99) - If you don't fit the enterprise program description then this is what you go for. Allows you to publish your application(s) on the app store.
I presume you should go for option b). Like everyone else.
Then you will have to choose between "Standard Individual" and "Standard Company". Which should be quite easy for you, will you be publishing you application(s) in the name of your company or the company you work for?
If you answered yes go for "Standard Company".
Else go for "Standard Individual".
And yes, you will be able to sell your application which ever options you choose, enterprise, standard individual or standard company. That is the whole point of the membership.