My girlfriend goes to a church that I believe is not right with God. They have choirs and music, and they don't take the Lord's Supper every Sunday. I really need to explain to her why you must take the Lord's Supper every Sunday and why you must only sing and not use musical instruments.

There is material on this website that should help you, if you are willing to spend time learning it. It is difficult to teach someone else when you yourself are not comfortable with a topic. Take a look at "The Lord's Supper" for general information. A prior question: "Why must the Lord's Supper be taken every week?" directly discusses why every Sunday.

In regards to instrumental music take a look at "Music in Worship" and "The Amazing History of Instrumental Music in Christian Worship"

But there are two other things you need to consider. Too many people try to fix problems in other people without first fixing their own problems. You and I will never be perfect, but each of us must understand that when we are involved in sin, anything we say comes out hollow sounding, even when we are right because our sins show that we are not sincere. "For with what judgment you judge, you will be judged; and with the measure you use, it will be measured back to you. And why do you look at the speck in your brother's eye, but do not consider the plank in your own eye? Or how can you say to your brother, 'Let me remove the speck from your eye'; and look, a plank is in your own eye? Hypocrite! First remove the plank from your own eye, and then you will see clearly to remove the speck from your brother's eye" (Matthew 7:2-5). Teaching needs to be done, and I want you to talk with your girlfriend, but do the best that you can to give the gospel its greatest impact. In order to effectively teach someone else, you first need to take care of your own problems. Then you will be in the position to lead others to Christ.

Second, unless these issues are resolved, you need to seriously thing about whether this woman can be your life long companion. If the two of you are unable to agree about religious practice, you will have established a foundation from which you will be arguing for your entire marriage. Too many ignore this. They think they can change the other after the marriage. But if a person is unwilling to change beforehand, why would they change afterwards? Some think that they can agree to disagree. They will go to their church and she to her own. Some survive that way, but it always breaks down when children come along. In which religion will the children be raised? Worse, the children get pulled back and forth between the disagreeing parents. One parent is constantly contradicting what the other parent says. Sadly, most of these children grow up rejecting all religion as a result.

See also:

Questions and Answers regarding Dating
Questions and Answers regarding Teaching
Questions and Answers regarding Music
Questions and Answers regarding the Lord's Supper