I’m a beginner in open source contribution and I would love to contribute to the p5.js repository. But the codebase is pretty massive and I don’t really know which is the best place to start. I would thoroughly appreciate some tips and guidelines on how to go about things in the beginning. I’m familiar with JavaScript and have done a few web development projects.
Hi,
Welcome to the community! 
To start contributing to the p5js project, you can check the Contributor docs.
Also Luisa Pereira’s videos about looking inside p5 (the link is from the contrib docs).
And finally a Coding train video about adding a feature : here