Good forum! Hopefully I'll be able to answer some of these questions seeing that I am majoring in Bible & Theology..even though you would have no idea.

The O.T. is before Jesus' coming and the N.T. is when Jesus is on Earth and then Heaven.  Not sure how they were brought together but there is a number of authors that wrote the books.

So, is it ever addressed anywhere in the bible why all these people felt compelled to write their stories? Like, does it say anywhere that God told them to write their stories and put them all together, or is it just this weird coincidence that it all happened?

And did the old testament have just one author, or are there multiple authors for that as well?

SIF, I can't answer all of your questions, but I do know that the NT was composed by many different authors.  Although I'm not 100% sure, I believe the OT was different authors as well.  David wrote the psalms.  I have no idea who wrote genesis.

The OT is what the Jews use for scripture.  Christians added to that after Jesus came along and Christianity was established (the NT).  Several hundred years after he walked the earth many of the writings that were about Jesus and his life were compiled and it was decided which ones were to be included as parts of the NT.  Some books, like the gospel of Thomas, were not included because it wasn't believed to be divinely inspired. 

That's my understanding of bible history. 

I'm not sure if it addresses why they write it.  I assume that God told them, but I could be completely off.

The old testament is filled with a number of authors.  Some of the books start off by saying who wrote them, and some don't. Some of the books are named after their author, one of those being the book of Joshua. Historians were able to backtrack it to some of the authors though because of who they talked about, what area they talked about and the style of writing.

I know that they believe the first 5 books of the O.T. were written by Moses

Ok, the following aren't necessarily my personal beliefs, but Christianity is a hobby of mine :)

The first 5 books of the Old Testament were written by Moses.  Even the part of his death, and after they reached the Promised Land.  He was pretty awesome like that.

Thruout history, there have been different conventions where Holy dudes got together and somehow decided what books went into the Torah (OT for Christians) and the NT.  And Catholics have more books than Protestants. 

SIF, I digress...despite taking a Foundations Bible study, I am still LOST!  (the origins of the Bible and all of its part still stump me..even more so than LOST)...so I will let another answer.

St Paul never intended for his letters to churches to become scripture, but they did. 

Oh, back to the Old Testament.  Moses is credited with writing the first five books, but there is also another theory out there.  (Are you going to ban me from this thread?  I might not shut up, lol!)  There could be a few different authors, known as J, E, D, and P. 

The Gospels could have been based on/inspired by the works of an author known as "Q" (no, not from Star Trek"). 

SIF--For Christmas one year, I received "The Archaeological Study Bible" (NIV).  It's a Bible, but it also has some of the info that you seem to be looking for.  Pictures of Holy sites, back ground info, it's pretty awesome.  You would have thought I got a pony or a Ferrari for Christmas when I opened it, lol.

Do the original writings still exist somewhere, or has everything pretty much just been handed down over time?

That sounds Awesome Sonja, I would probably love that too! I'm very curious about the history...

SIF--it's great, because you know you get some shows/documentaries that are slanted like "well if there's no proof, then it couldn't have happened" or getting too much into the interpretation of things.  This doesn't go into the meaning of the Scripture, it just adds to it, gives you different things to think about without judging. 

A lot of it had been passed down orally, which was a huge part of Jewish culture.  Some of the earliest documents found of Holy texts were the Dead Sea scrolls.