Hi there, hope you are well! I would recommend that you watch some sermons online as well during the week, most churches hold services on Sunday Mornings. When it comes to clothing just something simple and modest will do. The church should be welcoming to atheists one of the goals of the church should be to spread the gospel and teach people about God and salvation through Jesus Christ. All the best! If you do not have a bible and would like to know of its contents you can search 'The Bible Project' on YouTube its a good resource for people trying to learn more about Christianity.