Working on Linux - How to add a system call from the databases?


I am new to Linux. I want to add a system call to linux. But I don't know from where to begin.Can somebody please guide me through the whole process? From installing Linux to getting the kernel source code. I don't know where will I run or test or write the modified source code. Please help me with that also. I searched online for the linux kernel source code but it is giving me a lot of options of GIT files. What are they? How do I install them?

thank you.

Here are some good tutorials for adding system calls to kernel