That error means that something else is already reading the sources list, probably the Software Store or a background process doing some updates. Close it, or wait until any updates are done, and the "lock" will be released for you to edit those files.
For general advice, honestly I think @anon2532484 is spot on: play around and explore the system, what you can change and how to change things around, etc.
Many video or online blogs will list things that may or may not make sense for your use case, and you won't learn as much.
Maybe you want to go through some of the setups that people shared over at this thread, and try to replicate them yourself. This can be a good starting point to learn about the different components that are available in the settings, extensions, etc.