When I started learning to mod a couple of months ago I found this channel. As far as I can tell (I only watched the first couple of videos) it's not super bad, but there are several things that you'll want to change, such as the proxies we've already mentioned in this thread. It looks like there are a lot of concepts covered in the series, so it's probably a good starting point to teach you the more specific parts of larger concepts covered in the official documentation. Keep in mind that the tutorials are for 1.13.2, not the latest 1.14.2. The documentation hasn't been updated, either, but it's really valuable once you start to understand how modding the game works.
You're going to need a good grasp on the Java language in order to be able to do what you want - just copying/pasting code from the forums won't get you anywhere in the long run!
Are you using Eclipse to write/organize the code? If so, when you enter "gradlew runClient" in the terminal there should be a log output into the terminal.