Members can sign up directly, apply, be invited, gifted and/or full-service. It is important that you understand how all of these works.
First off, you have simple controls for people to enroll as a member by managing the sign up options on the membership program. You will find this on the membership program create/edit screen under the Member Signup section.
If you want people to sign up directly and instantly become a member, you can set the self-signup link for people to access. You can either share this publicly so it's visible on https://yoursite.com/memberships or you can keep it private so they need a direct URL.
You can also require the user to complete an application in order to become a member. In this case they will need to fill out the application where your team would then approve the application.
When choosing Full Service, you will need to setup members in the Control Panel.
One other way for people to become a member is by using the ecommerce engine. When using the ecommerce engine, if the product is set to active, then anyone can become a member regardless if the membership is inactive or not.
To control the ability for someone to purchase a membership, you will need to edit the status of the product.