I use Beep Box, as it runs on your browser and is free. Pretty simple. It's pretty similar to Bosca Ceoil, but without downloads. Just click buttons and it makes notes, and the UI is simple and you won't need any youtube tutorials.
If you want to go advanced, go with Musescore. It's free, but you need to register and download, and you need to know how to read sheet music. On the bright side, it also allows you to share your music, for input and as a way of publishing it. You may need a youtube tutorial though for setup. It takes longer, so I don't use it for Jams when I'm on a bad time crunch. It is much more advanced.