inequality - How to show $x_k in mathbb R, frac {sum{x_k}}{n} leq left(frac{sum{x_k}^2}{n}right)^n$?

Prove that, for arbitrary real numbers $x_1,x_2,x_3...,x_n$

$$\frac{x_1+x_2+x_3...+x_n}{n} \leq \left(\frac{x_1^2+x_2^2+x_3^2...+x_n^2}{n}\right)^n$$

What theorem would you use to prove the following inequality?
I would also like to know how to learn more about inequalities.Thanks