I'm very new to Japanese, and I was wondering when is it appropriate to use what alphabets? I've been practicing entirely in hiragana thus far, and I know that katakana is for foreign words and sounds, but how do you know when to use kanji?
Japanese is written using a combination of all the three sets of characters: hiragana, katakana, and kanji.
You should expect them to come together in an average Japanese text (meant for adults) and not in isolation or in exclusivity. Children start off reading and writing everything in hiragana because it is easy (if you look at children's books, you can confirm that is the case), but as they learn more and more kanji (Chinese characters), they have to use those instead.
Kanji are characters originally from China and are, easily speaking, used to convey ideas. In Japanese, they are used as the basis of each word while hiragana complements it by adding conjugation, etc.
e.g. 食べる（たべる to eat); 旅行する (りょこうする to travel)
Katakana are a set of characters that have a one-to-one correspondence with Hiragana characters. Some of them might look very similar, like hiragana か vs katakana カ. Essentially, Katakana are used for foreign or loan words. Furthermore, they are also used for technical and scientific words. In manga, a character using katakana for its speech bubble might also indicate he is a robot.
e.g. コンピュータ computer; アナウサギ word for a kind of rabbit (demonstrates scientific terms are written in katakana)
So in conclusion, you have to use all of them to read and write "fluent" Japanese. In my opinion, once you learn the Kanji, your reading becomes so much faster and convenient. Talking from experience, after having studied Japanese for more than four years, I feel rather annoyed if I have to read some text that only has hiragana (like children's books). It is much faster to read a single character (e.g 表す） than to read four characters just to speak the same word （表す is read as あらわす）.
Gonna try to summarise what each system represents:
Pure hiragana/kanji words are native Japanese, kanji came from China a long time ago; kanji often marks the start of the word, but not always:
元気 = healthy, lively
ある = to be (inanimate)
行く = to go
Some words have kanji, but are written as pure hiragana instead; some are/aren't based on the context in question:
可愛い -> かわいい = cute
Actually, you can write any word without kanji; it's just that some are more common, and doing it to much will make your writing look kinda cute:
捥ぐ -> もぐ = to pluck
Some words are in the middle, with it being just as common to use kanji, as it is to just drop the kanji and use pure kana:
持つ -> もつ = to hold
Pure katakana words are used for loanwords. Hiragana like る will be used if the loanword is a verb, in which case it's a godan verb:
プロジェクト = project
カー = car
ググる = to google
ウィキ = wiki
ウィキる = to look-up on wikipedia
Some loanwords are unique though, like using English characters:
Tシャツ = T-Shirt
There is an infinite amount of nuance in the Japanese writing system, just like how the English alphabet is just as much of a written/spoken mess; I struggled a lot at first, too; so don't worry, you'll figure it out as you get used to using it; you know, you'll learn to feel it; just need practice.
As for names: katakana is used used to make names like ancient, as katakana was the first Japanese script, with hiragana and kanji coming later. Names are normally all kanji, but if the names kanji is dropped by the Japanese government's list, they gotta use hiragana in it's place instead, at least for legal documents.
Sorry if my answer is messy, if you want me to clean it up a bit; or explain something, just ask!