A nice feature of the Bitsy tune tool is you can pick a scale (under mood) and limit yourself to that. I would start off with major (cheery) or minor (gloomy). That way any notes you add will sound in tune with each other. From there, you can build a simple groove, just adding notes on the beats where you want them. The sequencer is already divided up into a 4/4 pattern, so it's just a matter of playing around, adding notes and seeing how it sounds. Stick to one bar (16 steps) at first. Music is a kind of play, and it's key to just play around and try things out and see how they sound to you.
Once you have a basic groove that you like, try removing some notes. Give it some room to breathe. Then copy and paste it into a new bar. Move a few notes around on the new bar so it sounds a little different. Then just keep doing that and iterating. Before you know it, you'll have a whole song.
If you want to check out some example tunes, I made a bunch for my game Sleep Cycle. They all use different techniques. Feel free to download and copy those and mess around with them, remix them, generally do whatever.