When do you use katakana?
I'm new to learning Japanese and am having a hard time distinguishing between when to use Katakana and Hiragana. Any help in describing when to use one or the other would be much appreciated. Thanks!
As a general rule, use Katakana for names and words that are borrowed from other languages.
To add on to what others have said, sometimes you'll see katakana used for normal Japanese words when someone is perhaps yelling, expressing emotion or if they just want to give their speech some sort of flair. It's still mostly used for foreign words, though. You'd most likely see katakana being used for normal speech in things like entertainment--songs, manga, etc.
https://en.wikipedia.org/wiki/Japanese_writing_system (first paragraph)
Other people have listed things that are usually written in katakana. Foreign words, sound effects (you will notice this if you read manga), sometimes other things.
You won't always know what is or isn't an import word, or other thing that uses katakana, especially as a beginning Japanese student. That's okay. You'll pick it up over time. I think it's just one of those language things you may have to memorize the hard way at first, like genders of words in German.
Because there are no spaces between words in Japanese writing, it becomes really useful that there are three writing systems being used. It makes it much easier to visually parse sentences. Katakana stand out well because their simple, angular shapes are so distinct from hiragana or kanji. Once you get used to katakana, you'll probably appreciate that aspect.