First thing about orchids: there are many species from many parts of the world, so they all like a bit different treatment, but one thing is ALWAYS true.
Overwatering them will kill them faster than anything else. Their roots and leaves will rot very easily.
The main reason you will often buy a pretty orchid and it dies soon afterward?
The store/nursery where you bought it from overwatered it and killed most of the roots before you brought it home.
Whenever I buy a new orchid, although it's tempting to choose one with the prettiest blooms, I always get the one with the best roots because, like I said, they rot easily and unlike most plants, orchids do not just send out new roots. They only send out new roots from a new pseudobulb, so if their root system is rotten, it's tricky to coax an unhealthy orchid through until it's healthy enough to make new roots.
So don't be afraid to dig up the top layer of moss/bark/etc and check out the roots.
The first thing I do when I buy a new orchid is, AFTER it finished blooming, I re-pot it, and this is why:
See the stuff in my hand in the picture below? That's sphagnum moss, what baby orchids are grown in, but as they get bigger, most growers/nurseries just keep putting them into a bigger pot with more moss.
Baby orchids do not have pseudobulbs to support them (next post), so they need more moisture. Sphagnum moss is ideal for that.
Mature orchid roots need air. They need to dry out. Packing them in moss ensures that it's nearly impossible to tell when it's time to water them and the moss holds/maintains a ton of moisture -> so, perfectly ideal for root-rot.
What I do is, I take the orchid out of its pot and soak just the roots in room temperature water for about 5-10 minutes. This makes the sphagnum moss easier to pull out and the roots are springier so they are less likely to break.
Removing all the moss is a pretty tedious process. I also cut off all dead roots. Sometimes you are cutting off all roots except one or two, and that's fine. It's better to cut them off because the rotten roots will hold moisture and might end up rotting the healthy roots.
And sometimes you will cut off ALL the roots. This little guy has zero root system.
When I'm done, I'll repot the orchid in 90% orchid bark, 10% sphagnum moss, which means the medium doesn't hold very much water. A few are in peat/soil/vermiculite, but those are orchids with really small/thin roots.
I also pot them in clear orchid pots. This is good for several reasons. First, you can take the clear pot out of the ceramic/other decorative pot and visually see if it is dry all the way down to the bottom, which is when you water it. Second, you can visually see the condition of the roots. If they are too packed, you can see when you should repot them. Third, having a pot within a pot makes it easier to water them. My decorative pots do not have holes in the bottom, which is what I want because of how I water them. Notice the clear pots also have slits in them, this is because the roots need air.
I only water my orchids about once every 2 weeks. What I do is, I fill the pots with water (taking care NOT to get the plant wet) and let them sit and absorb water for about 5 minutes.
Then I lift the clear pot, dump the excess water out of the decorative pot, then set the clear pot back into the decorative pot. This way is super easy to water them without risking crown rot, and the orchid's roots will not be sitting in excess water.
Healthy roots look like this:
And you can see how big the bark is. This is a cattleya orchid. It and phaleonopsis orchids have the largest roots, so large bark is good. Also, these types of orchids actually photosynthesize with their roots, so loose medium where the roots can get out to the surface is a really good thing.
See these roots reaching for the light? And how they have green tips?
By the way, crown rot is when water gets in between the leaves, since orchids are jointed/layered, this is easy to do by mistake if you pour water onto an orchid instead of soaking just the roots. On a cattleya orchid, it looks like this:
See how black the plant is just above the new roots growing? And see all the dead roots? This is an orchid I'm rehabilitating, but it's strong, and the new root growth is a super good sign.