I would suggest, before doing this, NOT turning it on.    

Be patient. Get silica gel packets. Rice. Make sure you dry the piss out of it.    

Then do what dude said.

I spilled a little water on my keyboard, turned it off and propped it up in a V shape trying to get water to drain out. I tried turning it on later and was able to type in my password and have it start to log on before I got the prohibitory symbol (circle with slash through it). 

Will leaving it off overnight before trying to start it again or is it probably toast?

MBP ‘13 showing prohibitory symbol after spilling a little water on keyboard