This is a difficult question to answer for you specifically because it's about who you are as a person, and I'm not sure who that is. :)  I can give you some examples to get you started in the right direction.

You might say that you life is like a pencil: when it's new, it's unused and perfect, like a baby.  It gets sharpened--maybe that represents going to school and getting "sharp."  Sometimes your life is boring, and that's when the pencil gets dull?  I'm sure you could come up with something about making mistakes and a pencil having an eraser--or what happens if you use up the eraser.

You might say your life is like a river--some times it looks very peaceful and calm.  Sometimes it's rough with a lot of rapids and small waterfalls.  Sometimes you run into rocks--problems.  Sometimes the water is clear, other times it's muddy. 

You might say your life is like a bird:  sometimes everything is going well and you're soaring in the clouds.  Sometime you're grounded because the weather is too bad.

Think of the important things that have happened in your life and think of words that describe those things:  rough, easy, emotional, etc.  Then try to find something concrete (something real) that can help someone understand what your life is like.  That's the point of a metaphor--taking something that you can see (like the whole of a person's life) and making it easier to understand by comparing it to a real object that is easy to understand.

If you've every seen the movie Shrek, Shrek uses a metaphor when he says ogres are like onions--they have a lot of layers, and you can only understand them as you peel away the layers.