Read Genesis 1-3

Before the incident with the fruit, is there evil in the world (or outside of it)?

What is the nature of the first sin, and why was it such a problem?
(i no the first sin is eve taking the fruit but why is it such a problem?)

2 answers

It was the sin of disobedience. According to the story, God had said not to eat of the forbidden fruit ( metaphorically - knowledge). This can also be called man's desire to take responsibility for himself - self determination.
With this comes the ability for man to chose a course of action and therefore, he can make bad (sinful) decisions.
If man had not chosen to make his own decisions, but had lived "totally innocently" without knowledge, then he would have always been as a "child". That is why that choice has caused "problems". We ge to make mistakes, but then on the other hand we also get to make our own choices.
Good answer by Guru. To expand on it a little bit more, knowledge in itself isn't bad. The problem comes in with how we use knowledge.

Notice that after eating the fruit, they are ashamed of themselves. Before that time, the focus of the world was how beautiful the world is. After that, Adam and Eve looked at themselves and were embarrased. It is also a problem that we do not see ourselves as great creatures of God, but as something God messed up and needs fixing. Imagine how many problems in the world stem from this idea.

Matt