software installation - I am on Ubuntu 18.04 LTS, and is dual booted. I can't install Netbeans!

Netbeans didn't work at first because I didn't have Java installed and now, I have the JDK 10 installed and it won't open -,-. Before I installed JDK 10, I installed JDK 8, it worked fine, but when I can't do stuff after the installation because when I started the app, it says I have to disable the modules and stuff. I am really lost because I am new to Ubuntu, I used to be a Mac user and I dual-booted Ubuntu. I had trouble installing a lot of things because it requires shell (or Terminal) commands. ESPECIALLY Java! Why doesn't it have an installer like the one for Macs and Windows? But I didn't care so I took the matter bare-hand. Instead of fixing, I think I made it far-more worse! Please help me!
P.S. Now, Netbeans can't even start! And I read the logs, it said, " Updating from such a repository can't be done securely, and is therefore disabled by default."