It can be a homeowner's worst nightmare to face a plumbing issue. But take a few deep breaths. You've got this. Assess the situation:
Is water gushing anywhere? Or maybe it's just a tiny trickle that somehow found your floor?
Either way, you've got a plumbing emergency on your hands, and it's not going to fix itself. If you can see where the problem is, great. If not, it's kind of a wash, because either way, the next step is to shut off the water.
Find the main shut-off valve and turn that sucker clockwise until it won't go anymore. If you can't find the main valve or it's just too stuck to turn, check to see if you have the shut-off valve for the sink or for the toilet.
Once the water supply is off, it is time to contain the problem. That means using towels and/or buckets to sop up any water that might otherwise go flowing and creating a mini-red-carpet tsunami (think: sea, not stream) along the floors and into your precious furniture. This part is actually quite crucial to avoid damage not just to the floors but also to any pieces of furniture that might be affected. While doing all this, it is also an opportune time to check for other leaks that might be happening in the room near other plumbing fixtures and appliances.
When you have a plumbing emergency, the next step is to call a professional plumber. Most of us can't handle a major plumbing problem on our own, and trying to do so can result in more damage and more expensive repairs later on. Plumbers are the obviously trained professionals who can deal with all types of emergency situations. But even when plummeting in the water world, you can't just call anyone. You need to call the right person.