Do I need an injection or anything before an MRI scan?

Not usually. Most MRI studies are done without contrast. The studies done with contrast usually include an injection part way through the study. Occasionally, if a person is claustrophobic, the Doctor may give you a shot before the study to help you relax. Discuss details with your Doctor, or the staff of the MRI center before you go there.
Not before ... However, depending on the reason for the scan, a contrast agent may need to be injected into a vein in the arm about 2/3 the way through the test. These post contrast images show how blood flows to different parts of the scanned area. Contrast is more often used for head, heart, abdomen and pelvis scans, and rarely used for musculoskeletal (knee, shoulder, etc) and spine scans.