I dropped my phone in the toilet, and after turning it off for 12 hours the screen is still glitchy and the screen sensor stopped working. Any ideas on how to fix it or how much it will cost?

Water damage often damages many components, so the phone needs to be replaced. It would be $99 if you have AppleCare+ and it would count as one of your two accidental damage incidents. If you don't have AppleCare+, it would be $299.

Other iPhone Repairs - Official Apple Support

An out of warranty replacement is $299. Make an appointment at the genius bar.

As for why it happened... electricity and water don't tend to play well together...

jtl33 wrote:

Any ideas on why this happened

Because you dropped the device in water, and water and electronics do not mix.

jtl33 wrote:

what I can do to fix it, or how much it will cost? I have an Iphone 6s

Make an appointment at the Genius Bar of the nearest Apple store to have the device replaced. Water damage is not covered by warranty, so the cost is on your. Check this link to see what the cost would be where you are located. Other iPhone Repairs - Official Apple Support