I understand Shanghainese but I learnt it in an immersive environment. I lived in Shanghai for many years. My husband is Shanghainese and speaks Shanghainese with his parents. So I heard it on a daily basis. How good is your Mandarin? Most resources for Shanghainese are in Mandarin.
This channel is a start: https://www.youtube.com/watch?v=QLtKkbTxKfI
This is an introduction to some Shanghainese slang in English: https://www.youtube.com/watch?v=zTe7gC5mk8g
Since Shanghainese is a spoken and not written language, you should look for audio and video material.