To become a Lutheran, only Baptism and instruction in the Christian faith is required. If you are already baptized in the name of the Father, Son and Holy Spirit, it will be necessary only to attend a membership class in a Lutheran congregation and thus signify your desire to become a part of its community. Active members of other Lutheran congregations usually need only to transfer their membership.
For more information visit the website of the Evangelical Lutheran Church in America by clicking this link http://www.elca.org/questions/