Well, first, open it up, and remove the battery. Water can do serious damage to it.
Then you need to remove anything else you have connected to it. Especially a power cord. And leave the laptop open and in a place where it can dry. You need to leave it for a long time. Days. Yes I do mean days. It will take a really long time for it to dry out, even if it had been less water, however with that much it is going to need a lot of time. I would give it a 6-7 days at least. Somewhere good and safe and allow it to really dry out.
After that time, try putting the battery back in and try starting it up. But don't try it sooner, nor leave the battery in. Both could cause major damage.