My coworkers asked me to print some things, but I don't want to.
For what reason are you printing things for your coworkers?
I am not sure, it's really costly and not doing me any favor.
Put your foot down. You're not getting paid extra for doing all those things. They will just keep asking you for help without any monetary compensation.
I know that but I feel like I'm being a bad coworker.
Standing up for yourself does not make you a bad coworker. You need to be assertive so they know they can't expect you to do their jobs, especially without compensation.