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