You become a member of the established church by being baptised into it, usually as a child, but otherwise as a teenager or an adult.
If you wish to be baptised, you will need to speak to a pastor. It is the pastor who leads the church service on Sundays and thus the pastor who performs baptism. Together you and your pastor will find the best way for you to prepare for your baptism. For teenagers, preparation can take place as they join their friends who prepare for confirmation. For adults, preparation can take the form of individual conversations with the pastor or a course taken together with others who are also considering baptism or who want to know more about God and what it means to be a Christian.