You should not be nervous at all.
First, you are not allowed to wear any metal, including jewelry. You will change into hospital gowns and then they will have you lie down. The bed you are in will move you into the machine, kind of like a tube. They control it from just outside of the room you are in, but you will be able to see them through a glass window most likely and you will be able to communicate the entire time through speakers.
The person running the test will tell you what is going to happen before it happens. Including telling you that they are about to start scanning, when they are about to start, and you will hear a lot of clicking and banging noises while being scanned. You have to stay very still for the MRI to be accurate. They will run the test for a minute or maybe a few minutes, and then stop. They will check you are feeling ok, give you a chance to move a little before you need to be still again, and then most likely start another scan where the banging and clicking will start again. Probably they will repeat that process a few times. There is absolutely no pain, nothing is touching you, you are just lying there while the machine takes pictures. Again, you will be able to talk to them at any point if you get nervous.
If you have some claustraphobia you can ask for medication ahead of time. I advise you to close your eyes during the process and just rest.
The whole test takes about 20 minutes.