Years ago I modified the Postgresql source code (8.xx). It's something I was terrified of doing. But once I got in there and started poking around, I realized it was just ordinary plain-vanilla C code. Not C++. Just C code. With my local copy, I started to hack pg_dump to do something special that we wanted at the time. Even after 30 years of coding, I'm not that especially good of a programmer. But I ended up gettin…
I can relate to this as well. I will share couple of things just to add to this.
I have ~8YoE now. When I was at my first job, I did not have any formal CS degree, I had completed bachelors in commerce and was struggling with masters in computer applications (had year drops). My first job was in a company started by ~7-8 ex-veritas folks, all of them being hardcore system developers. I had dreams of being the same like them some day (yet to happen). ~1 year in this job and I shared my aspirations to become a system developer - I was given a task, implement persistent ram mechanism, something that will persist data in the RAM even after soft reboot, without dumping data on hard drive, using Linux kernel. "what to do" (trick/technique) was told by them, how to do it was left for me. It took me 4 days (2 weekends) to complete this. Over first weekend I learnt how to get Linux source code, add custom syscall, compile kernel etc. On second weekend I actually got to go through the code, find places to add patches, test etc.
I was also afraid before starting and my then boss had said few things like .. "because you think folks sitting in the west are something special, they are not ..", "whole thing is man made. If one man can do it, so can you". It was a matter of going through the code and understand. Do things repeatedly without giving up. Spend long hours, take notes. Once you have the context and that code is running in your head - you get what to do!
I also did the something similar few years after this. I was learning Go and wanted to do something better. I got into delve codebase (debugger for Go) and I patched it to work for cgo binaries. Its a small patch but for that I had to learn delve's architecture + what is dwarf standard and stuff. Context was pretty huge compared to what I needed to patch it. Same story though, I was little afraid thinking - omg! it's a freaking debugger, how am I going to understand all this to make changes. But in fact they are just the same constructs.
Go ahead and jump into some project you use, it helps understanding codebase faster. Read Read Read. Give yourself time to learn the codebase. You will definitely be able to contribute "properly" for that project. Don't be afraid :)