Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech , or of the press; or the right of the people peaceably to assemble and to petition the government for a redress of grievances.
Basically the first amendment is saying that you can say or write anything you want.Also you can choose any religion you want. Another thing is that if you have a problem with government you can speak up and tell them.
Neither a state nor Federal Government can set up a church. Also can't pass laws which aid one religion, aid all religions, or prefer one religion over another.
Supreme Court Case
Lemon Vs. Kurtzman (1971)
The court established a three- part test to determine whether governmental regulation violated the establishment clause.
The court struck down laws that used public funds to help pay teachers in parachial schools.