If you want to write a bot from scratch for free on Glitch, you can remix this starter project A new Discord bot starter, no pinging needed (of mine, full disclosure). There aren’t any tutorials for how to do anything this way, so you’d have to read the reference materials on the Discord gateway and Eris’s REST APIs.
Or you could write a bot using an existing library, in which case there are many tutorials. If you do it this way, when it comes time to run the bot, you can either buy a Glitch membership or export it to another hosting service.