We have an introduction document for new students here:
https://joplinapp.org/gsoc2020/
See in particular the section “Instructions for students”, which will tell you how to get started.
Then have a look at CONTRIBUTING.md and BUILD.md as they contain very important information about the project and how to contribute.
Finally, please create a new post on the forum to introduce yourself, and if something is not clear in any of the documents above, please let us know!