If liquid spills onto your Mac, shut it down and disconnect power and accessories. A spill through the keyboard or into the computer needs assessment by Apple or an Apple Authorized Service Provider. A dry-looking surface does not tell you whether the inside is safe to power on.
What to do immediately
Shut down the Mac and disconnect it from power and accessories.
Keep it powered off. Do not reconnect power or try starting it to see whether it works.
Do not push paper towels or other objects into openings, use a hairdryer or other heat source, use compressed air, or put the Mac in rice.
Contact Apple or an Apple Authorized Service Provider promptly for an assessment. Explain where the liquid went and whether you need help preserving data.
Days? I need to get to my files now!
If you have been using cloud storage such as Dropbox, Google Drive, OneDrive, or iCloud Drive, you may be able to access your files through an iPad, iPhone, or another computer.
What to do next
Arrange an assessment with Apple or an Apple Authorized Service Provider before attempting to use a Mac with internal liquid exposure. Apple's liquid-detection instructions address wet USB-C connectors on supported models; their drying and retry steps do not establish that a keyboard spill or internal exposure is safe after a fixed number of days.
Unfortunately, most Macintosh computers built since 2017/2018 do not have removable storage, which means that we can't easily get at your data if the computer won't turn on. If the data you have on the computer is very important and is not backed up or synced to cloud storage, you may be looking at $1,000 or more to send your computer to a data recovery specialist such as Drive Savers. If you need to speak with me to review your options, please book an appointment.
Check your insurance
It's worth considering what insurance might cover your damaged computer. Depending on how you are covered, you might be advised by the insurer to handle the situation in a particular way so give them a call.
If you have AppleCare+, you might be able to get your computer replaced at the cost of the $299 deductible. AppleCare+ covers water damage from spills but not from submersion which could be a judgment call by the Apple Genius. If you aren't sure if your computer is still under AppleCare+ you can look it up. Then contact Apple to schedule an appointment.
If you purchased your computer with a good credit card in you might be at least partially covered by purchase protection or an extended warranty. Contact your issuing bank to find out what they might be able to do.
Traditional insurance such as homeowners, renters, flood, or business insurance might help out depending on the circumstances of the water damage and your specific policy, but if they have a high deductible, this might only be a viable option if you have an extremely expensive computer or if you have water damage to more than just your computer. Generally, insurance will not cover accidental damage if you knock a glass of water on your computer, but they might if it was caused by a leaking pipe or a flood. But it's probably worth consulting your insurance company.
Before you spill water on your computer
If you're finding this article now you're probably already at the point where you've got a problem. But in the future, these tips will help you to avoid or at least more gracefully recover from water and liquid damage:
Always keep a backup of your computer. I highly recommend Time Machine. This works with any USB drive with sufficient capacity (but I have specific recommendations). Just plug it in, configure it in System Settings, and any time your backup drive is connected your computer will back itself up. You can't leave it plugged in too much since it will just back up as you work. If you unexpectedly damage your computer this can be plugged into a new Mac and within a few hours, you'll be up and running with the same files, software, and settings. But of course only since you last backed up your computer. If you only back up once a month you might lose the past month's work. Cloud backups can be useful but they aren't as easy to recover and they don't back up your software and settings.
Always try to purchase valuable items with a good credit card. Purchase Protection and the Extended Warranty are huge perks that few people take advantage of. Depending on your terms you can sometimes get 1 to 2 years added to the warranty, which usually includes the manufacturer's extended warranty such as the 3-year AppleCare+. So best-case scenario you might be covered for 5 years total. If you rotate cards for sign-on bonuses be aware that canceling a card also cancels the protections for purchases on that card.
Avoid using a standard mug or glass of water where you work. I'm a fan of Contigo's travel mugs and water bottles, which require you to hold a button when you drink. It's not as arduous as it sounds; the button is placed where you would naturally put your thumb when you grip the bottle. As long as you have no trouble with your grip these should work well.