Let X be an exponential random variable with mean 1. Find the probability density function of $\displaystyle Y = -ln (X)$.

Can someone walk me through how to do this?

Exponential random variable has a density function $\displaystyle f(x) = \lambda e^{-\lambda x}$. In this case $\displaystyle \lambda = 1$. How do I find that density function of Y?