A bathroom is a great beginning organizing project, because there’s not a lot of space, and usually not too much confusion about what belongs there. If you are luck enough to have a medicine chest or drawers, you can usually work out a few sections: his, hers, backup supplies (of soap, shampoo, and toilet paper,) and first aid.
If you are not sure what to keep on hand for first aid, here is a helpful list from the Mayo Clinic:
All of this will probably fit in one drawer or container.
Something waterproof and lockable is best. Just make sure you can find the key quickly when someone is bleeding.
Of course, there is no need to lock up the bandages. However, if you have a little person in the house, lock up the medications (including vitamins) or put them up out of reach. (Calling poison control is not fun. I may know people who have had to do that in the past.) If you don’t have room in the bathroom, the kitchen or linen closet will also work; it depends how often you need the first aid kit. Adults might open it only a few times a year. Families with active children may need it a few times a day.
I have seen some lovely organizing systems with separate boxes or bins for pain relief, cuts & scrapes, upset stomachs… but unless you’re running a clinic, I can’t see needing that much.
If you have vitamins or medication that you take every day, the kitchen may be a better location for those things. Having them beside the coffee cups may remind you to take them in the morning.