Desu です is a Japanese copula. That means it's a word that grammatically links subjects and predicates. It's usually translated to English as "to be" or "it is."
But knowing what it is doesn't mean you know how to use it.
In almost every aspect of Japanese language and culture, there are multiple levels of politeness. Some say there are three general levels. Others get more specific and (terrifyingly) claim there are nine.
It all depends on whether you know the other person well, how old you are compared to them, and a dozen other things that even Japanese people can't figure out. So don't worry about it too much (right now).
With formality and politeness in mind, let's look at desu. There are polite ways to say desu, and yakuza/gangsta ways to say it as well. Here's the three you need to know:
Most Informal: da だ
Normal Situations: desu です
Formal Situations: degozaimasu でございます
When You want to act like a Samurai: degozaru でござる
da だ:
This is the most informal version of "desu." It's short, easy to say, and used amongst people who know each other fairly well (friends). Just like every other version, it's used interchangeably with "desu," and is fairly common in speech.
desu です:
This is the most common form of "desu" (duh). If you don't know which one to use, then use this one. It's fairly safe in almost all situations (unless you're talking to the emperor, or something, but that's when to pull out your gai-jin card and beg for mercy), and is used in formal, neutral, and informal situations. If you've just met someone, you should be using "desu." I think most people would be weirded out if you start using "degozaimasu" on them.
degozaimasu でございます:
This is the most formal version of "desu." You've probably seen me use it in the beginning of some of my videos ("Koichi degozaimasu")…that is only because I am using formal broadcaster speech (if only for a little while) and talking to a ton of people I don't know. Of course, don't pay attention to the rest of the video, where my Japanese degrades and I start talking like a mixed up gangster. You would want to use "degozaimasu" when you are in a very formal situation, if you were in an interview, if you are talking to a customer of yours, or if you are a train operator ("tsugi wa…tsugi wa kyouto eki…kyouto eki degozaimasu…" Sound familiar?)
degozaru でござる:
So, those were a few ways to use the word "desu" in a sentence. There are several others, which I have failed to mention. Why? Because you shouldn't use them. Stick with the first three (though, mostly the first two) until you've really got them down and feel comfortable using them in different situations. Watch how people use them on television, or ask your Japanese friends. This is one of the simplest examples of Japanese levels of politeness. Know that there are many many more, and hopefully you can figure out this one so you can start moving on to the others.