Chrome Music lab is definitely a really good place to start. You can get the hang of it pretty quickly and it will be really cool! After you are comfortable with that Beepbox.co is my next recommendation. It is similar to Chrome Music lab but you get a lot more freedom and with the help of some tutorials you'll be able to learn the basic idea of making a song. You can also run it in your browser. This is how I learned to make music. Technically you don't really need to learn any music theory to create your first song, but learning some basics like what are melodies, chords can help.
Don't really think about it too much and just try making some stuff. I'm sure that you'll love it!