Paul's concern in writing to the Galatians is the sufficiency of faith in Christ for our salvation. After he had preached his gospel of grace in Galatia, Judaizers (those attempting to force Gentile Christians to obey Jewish customs), had come through teaching the Galatians that their faith in Christ was not really enough. Instead, these new Gentile Christians would also have to follow all the Old Testament covenant laws (circumcision, kosher, ritual washing, temple sacrifice, etc.)
Paul is adamant that obedience to the law is not what merits our salvation. If the law could do that, then there was no reason for Christ to be slain. Instead, Jesus took the curse for us by His crucifixion, being "hung on a tree."
Paul goes back to Abraham and traces the ancient teaching of faith (which came 430 years before the giving of the law). It is through faith that Abraham was reckoned as righteous. It is through faith that Abraham's children (that's us - if we live by faith) are justified.
Is the law evil then? No, Paul says that its role was limited and for a time, to show us our need for Christ. The law acted as our schoolteacher, even our jailer, locking us up until we should be freed by the arrival of Christ. But it is no longer binding on us now that Christ has come.
Want a great verse to memorize? Try Galatians 3:26: "For in Christ Jesus you are all children of God through faith."