As you know water and electronics and electricity are not a good mix.
The impurities in the water causes corrosion and provides circuit paths for the electricity which were not in the laptop’s operating design and could damage the components.
First do not turn on your laptop and then you need to remove the battery as soon as possible from the laptop to minimize any further damage.
Then you need to dis-assemble the rest of the laptop and clean all the affected parts using Isopropyl Alcohol 90%+ (available at electronics parts stores) to remove all traces of corrosion and water. Do not use "rubbing alcohol" as in some cases this is only 70% IPA or less, can contain scents and is not as effective. If you do check the label to verify the amount of IPA
Here is a link that describes the process.
Electronics Water Damage
If the water spilled onto the keyboard you may have to replace the keyboard as sometimes you can cause more damage than what you had when attempting to repair it
As always with electronics, especially surface mounted pcb be gentle when handling and especially when brushing away the corrosion. You do not want to remove any components from the board.
Hopefully after you have done all this the laptop might possibly work correctly again.
Here is a link to the ifixit guide(s) for repairing your laptop.
Asus Zenbook UX303L Repair
If this process seems too daunting, take your laptop to a reputable, professional laptop repair service, experienced in liquid damage repair and ask for a quote for a repair. If you decide to do this, do it sooner than later.