Where do you go once leaving the plane? Are there signs and so forth?

That depends on several things.

In most airports, most planes will drive to a gate after landing, such that when you leave the plane you'll be in the airport, and unless it's a very (you probably have to go to the extreme) small airport there will be signs telling you what way to go when you're arriving. In all the airports I've been in the signs have been in both english and the local language.

If the airport you arrive at is busy (there can be a lot of reasons, they don't really matter here), the plane might park at a "remote" (to the airport building) spot, then you'll leave the plane, go down a set of stairs and into a bus (there will usually be several as they typically don't have the capacity for all the people on the plane) that will drive you to a gate at the airport, at which point you're in the same situation as above.

As the door(s) from the plane are only wide enough for one person, the passengers from each arriving plane will quickly be spread out, so there won't be a big crowd to follow, but if it makes you feel more comfortable you can probably focus on a couple of the people who leaves the plane just before (or after you, the jet bridge leading from the plane into the building should give you plenty of time. But be aware that they might be transiting (i.e. they have to catch another plane, and won't be going the same place as you are) and don't make the feel stalked.

Yes, some planes have more doors used for boarding (and deboarding), that is mostly a question about speed, as mentioned above the doors aren't very wide, so using multiple allows the airline to get people in/out faster. It doesn't really affect the (de)boarding experience.

Edit: Reading what I wrote, I got to think that there's way more small airports than large, and at those the common case might not be that the plane drives to a gate with a jet bridge, so some of my "most"'s shouldn't be taken too literal. Also I forgot to mention that the jet bridges might not fit on smaller planes, so if you're on such a plane and there's room, it might go close enough to the airport that there won't be busses, and you'll have to walk from the plane to the gate, but we're talking very short distances here.

Following the crowd is always a good thing. Although the crowd might not be always going where you are, the statistics are in your favour. There are also plenty of signs, so unless you're departing from/arriving in a really crappy airport, you won't get lost.

As for multiple doors, it is usually on larger airplanes. It helps with the boarding: First/Business class passengers at the front door, economy passengers at the door more or less in the middle.

Usually you are not allowed to "get lost" until you are inside the building, so you would be stopped before wandering of. Inside the building you should be fine unless you pass the "no re-entry beyond this point" which usually is clearly marked. Somewhere between those points should be the baggage claim. And since they want a nice flow in the airport it is usually hard to miss.