I am interested in learning about the congruence relations satisfied by the coefficients of modular forms, in particular I am interested in learning more about $\tau(n)$, the coefficients of the weight $12$ modular forms $\Delta=q\prod_{n\ge 1}(1-q^n)^{24}$. I am aware that it satisfies some remarkable congruences. Recently I got to know that these mysterious congruences were explained by Swinnerton-Dyer in light of the existence of the following representation(which was conjectured by Serre and proved by Deligne):
\begin{equation*}
\rho_{\ell}:\text{Gal}(\bar{\mathbb{Q}}/\mathbb{Q})\rightarrow \text{GL}_2(\mathbb{Z}_{\ell})
\end{equation*}
with $\text{tr}\rho_{\ell}(\text{Frob}_p)=\tau(p)$ and $\det \rho_{\ell}(\text{Frob}_p)=p^{11}$ for $p\ne \ell$.
I want to learn more in this direction and I found the book "Abelian $\ell$-adic representations and Elliptic Curves" by Serre. But this is not an easy book to read and seems to require some non-trivial pre-requisite(such as algebraic groups), which is currently beyond me.
My background: I am familiar with basic representation theory but have never studied Galois representations specifically. I am familiar with basic undergraduate algebra(including finite Galois theory). I also know basic commutative algebra and algebraic number theory. I am also familiar with the basic theory of compact Riemann surfaces. I am currently learning some geometry from Vakil's FOAG.
What would be a reasonable road map from my current state to understanding "these things"? Any advice would be appreciated.