The most challenging aspect of MAT137 (at least according to a majority of the students) is learning to read, understand, and write proofs, as well as rigour and formality in general. In the course we will not assume that you have any familiarity with proofs. Nevertheless, I get contacted by many students who "want to get a head start" and start learning proofs before taking the course. While this is not necessary, here is a list of resources that you may find useful.
Notice that reading proofs or watching somebody else write proofs will not help you by itself. The only way to learn to write proofs is to get your hands dirty. You need to practice writing your own proofs (which will likely be bad at first) and get feedback on them.
If you have other suggestions that could be added to this list, feel free to let me know at "alfonso (at-sign) math (dot) toronto (dot) edu". Thanks!