Before I forget, remember that sending nice bug reports is very useful Try to include obvious reproduction steps, and be polite. Try to see how they fix it, and if you find a similar problem in the future you can try to fix it yourself. (Or just send another nice bug report that is very useful!)
Don't be shame of small fixes, like typos in docs or comments. Until you understand what are the secret weird rules of the mantainers is better to start with something short.
For starting, fixing bugs is better than new features. A bug is a bug, but a new feature may not be in the roadmap of the mantainers. Don't spend more than 2 days writing them, you are never sure if they will merge it. If they don't like it, you wasted only 2 days.
For longer bug fixes or new features, ask the mantainers before doing a lot of work.
The project is huge. My recommendation is to concentrate in a part you like and ignore the rest. Try looking at the closed PR and find a few that you consider interesting and take a look which files they modify. Look at those files and try to ignore the rest. Also look at the commit history.
This project has a lot of dependencies, so my guess is that you will need a few weekends to be able to compile it. You will have to "sudo app-get install whatever" a few times. If you get stuck, ask in the mailing list/slack/discouse/discord/issues or whatever they use for communications. Also try to explain what you have done, what is the error and be polite.