I am a teenager and in a few weeks i have to have a full body mri, under general anaesthetic. first of all, what is the process?

Same as surgery: First you will have the standard asa monitors placed (bp, hr, pulse ox). Once your vitals are obtained and are stable you will be put to sleep with a induction anesthetic. Once your sleep they will place lma in your airway . They could also put a endotracheal tube but most likely it will be a lma. You will be kept slept with gases (volatile anesthetics) . Once the MRI is done you will be awake.