If you want a really good treatment of it, David Tong has great notes and some old lectures (480p era..) up on YouTube. The basics are not that hard, but it gets very advanced and difficult very fast. The thing to keep in mind with non-interacting field theories is that everything boils down to creation and annihilation operators which are really just elements of a Lie algebra. If you feel like you need a treatment of quantum mechanics beforehand, Shankar has a good, roughly mathematically rigorous introductory text on the subject. QFT is basically functional analysis and Lie algebra (and similar structures like Lie superalgebras) put together in a fairly messy way haha.