What is Matthew 18:18 saying? It sounds like we can make up the rules instead of God. And why does Matthew 18:19 say God will do what we ask of Him?

This is Jesus providing guidance as to how the coming church is to deal with someone exhibiting sin within the context of the fellowship of the church. He says, If your brother sins against you, go and show him his fault, just between the two of you. If he listens to you, you have won your brother over. But if he will not listen, take one or two others along, so that ‘EVERY MATTER MAY BE ESTABLISHED BY THE TESTIMONY OF TWO OR THREE WITNESSES.’ If he refuses to listen to them, tell it to the church; and if he refuses to listen even to the church, treat them as you would a pagan or a tax collector. I tell you the truth, whatever you bind on earth will be bound in heaven, and whatever you loose on earth will be loosed in heaven. Again, I tell you that if two of you on earth agree about anything you ask for, it will be done for you by my Father in heaven. For where two or three come together in my name, there I am with them”—Jesus Christ, Matthew 18:15-20 (quoting Deuteronomy 17:6, 19:15). The essential idea is two-fold: one of progressive inquiry and response, and one of reflecting the will of the Father. Simply put, if someone is insistently continuing in sin, and this sin is confirmed by the church family, eventually that person needs to be put out of the church so that they can have no influence within the church. Seeing them, in Christ’s words, as a “pagan or a tax collector,” is not as harsh as it first sounds: It simply means that they are now relegated to being the object of that church family’s evangelistic efforts, reaching out to them with the gospel of Jesus Christ as they would anyone else. The idea behind church discipline is always restoration. It is not designed to be punitive in nature.
The idea of binding or loosing has to be seen in the context of agreement on earth with the doctrines of heaven. If someone is declared by the church to be living in sin, that declaration is being made in concert with that acknowledgement already being made in heaven. Letting that person leave the church family due to that sin is again being done, as the Lord’s prayer states, on earth as it is in heaven—Jesus Christ, the Sermon on the Mount, Matthew 6:10b. And likewise, the restoration of someone and returning them to church fellowship again is mirroring an understanding already being in effect in heaven. We are not making up any rules; we are simply reflecting both the acknowledgement and leading of the councils of heaven.
A little bonus: A beautiful picture of intimacy with Jesus is created in the last line of this text, where He talks about two or three coming together in His name, that He will be there as well. In Jewish tradition, 10 men (called a “minyan”) were required to be able to convene a synagogue, or even to conduct public prayer. Jesus is requiring a far lesser number to conduct His business but, in the context of this passage, Jesus is telling us that He will be present with us, accompany us and guide us in matters of church discipline when we seek Him leadership and His wisdom. This is not a promise that God will do whatever two or three believers ask in general. That is a violation of the context of this passage.