Mites in a bathroom are a common problem because they like moist areas (especially good after a shower) leaves the room nice and moist. You can get rid of the mites by getting Mite spray at any home/garden store that will kill them.
Your bathroom is very humid and this is causing the mites. You need to get the humidity levels down and the mites will then decrease or go completely. Install a fan if you can or keep a small window locked on a latch to allow air in.
What kind of mites are they there are about a million different kinds and how they are gotten rid of depends on what kind they are and what is causing them.
I would call an person that can help me get rid of the mites without costing me alot. I would call a friend or family member to find the source of were their coming from. I would use a chemical at the source of were the mites are coming from.