Term/Phrase for telling something including necessary context

Let's say I want to tell someone a story, but in order that he'll be able to deeply understand it, I need to tell (or better - start with quite a lot of) certain additional facts, incidents, whatsoever.

How would this be expressed usually?

Edit: To clarify here's a more concrete situation:

Let's assume someone is in severe problems concerning several aspects of his life. Now he calls a friend tells him about the situation he is in, but to make him understand he needs to start with things which happened long ago already, e. g. that in his youth he once had a certain incident and then moving on from there adding facts which will lead to a proper understanding of his situation.

The conversation could start like this

Joe: I have some serious problems, Peter.
Peter: What is it?
Joe: I will tell you, but I have to ...
Peter: It's fine, go ahead.

Aren't you simply giving him some background information?

Background: the circumstances and events surrounding or leading up to an event or occurrence.

First, I need to give you some background.

Don't you need to provide a preamble - setting out the background to your account?

Or you could say "Let me give you the story so far".