When you commit your heart and life to Jesus, you become a "new creation" and a citizen of a new kingdom that includes followers of Jesus from every tribe, tongue, and nation - past, present, and future. After becoming a Christian and being baptized, participating in God's mission through a local church is the next step.
A "church" is a local assembly or community of these believers who trust in Jesus as their Savior and Lord. Believers should be committed to gathering regularly for teaching, fellowship, prayer, and to spreading the good news about the kingdom of God - that it is available to anyone who repents and puts their faith and trust in Jesus' life, death, and resurrection for the forgiveness of sins.
Being made up of people means that no church is perfect, but that does not exempt us from doing our best to obey God's Word. The New Testament is filled with commands and encouragement to be committed to and to sacrifice for the "Body of Christ" - the term used to identify this eternal and global community of believers (Ephesians 1:22-23) - including local churches.