Can you put in tampons even if you don't get your period yet, just for practice?

Yes, you could. However you have to mimic conditions like menstruation. That means your vagina needs to be moistened. You can do this by slathering the tampon and your vagina with KY Jelly. Otherwise, if you insert a dry tampon into a vagina without extra moisture, it's gonna chafe. You shouldn't practice wearing the tampon, though. Once you get it in there, you should take it out. Otherwise, you can needlessly put yourself at risk of toxic shock syndrome.