I'm an MRI radiographer, happy to answer any questions you may have!
When you arrive for your MRI you'll be asked a series of screening questions to ensure that it is safe for you to have the scan, e.g. any previous surgeries. You'll be asked to change out of your clothing & given a gown/scrubs to wear to ensure there's no metal going into the scanner. You'll normally be able to keep knickers on but your bra will probably need to be removed. You'll also have to leave all personal possessions in a locker.
The radiographer can explain the scan to you if you ask them before you go in. It is quite common to scan pregnant women over 12 weeks so I'm sure the staff will be able to put you at ease.
As pp mentioned the scan is very noisy, lots of loud banging. You will be given earplugs & depending on where you have your scan you may be able to listen to music through headphones, although when the scans are taking place you will still hear the noises of the scanner. You may also experience some nerve twitching which is normal, doesn't happen to everyone but if it does it's nothing to worry about.
You will be given a squeezy buzzer to hold in your hand, if at any point you want to stop or feel uncomfortable you just give it a squeeze & the scan will be stopped.
It's very important that you stay as still as possible during the scan to get the best images. I always advise patients to close their eyes & try to relax as much as possible. Good luck & I'm sure you will be fine! x