Making a sequence of random variables converge to $0$ almost surely

Question

Show that for any sequence $(X_n)$ of random variables there exists a sequence of constants $(A_n)$ such that $\frac{X_n}{A_n}$ converges to $0$ almost surely.

My attempt

I choose $A_n$ such that $\Pr\left(|X_n|>\frac{A_n}n\right)<\frac1{2^n}$ and I apply Borel-Cantelli lemma.

But I do not understand how to apply Borel-Cantelli lemma.

Thank you.