Let $X_1, X_2, \dotsc , X_n$ be a random sample from a distribution having probability density function (pdf)
$f(x|θ) = θ e^{−θx},\quad θ > 0, x > 0$.
Derive the likelihood function for $θ$, maximum likelihood estimator (MLE) of $θ$ and its asymptotic distribution.
I don't understand this topic very well, how can I derive the likelihood function for θ