New here and to Japanese, I have some questions! :)

Hello! I'm new both to the language and to the sub. I've been reading through some topics along the week here.

I study Japanese by myself, because I want to both read and talk in Japanese. I know it can be hard, but I just love learning languages and about the culture of the people in question.

I have 2 main questions about some things I believe are truly the basic stuff.

First: how to pronounce the らりるれろ? Some say is like the R from Spanish, in which you tremble the tongue a little; others say is like L… other say it can be both depending on ???

So, there's some rule about the sound, like before this and that letter (あいうえお) it sounds differently?

Second: I saw that Katakana usually are used by foreign words or things that aren't Japanese. But I already saw in some places that cat in Japanese (ねこ) also written like (ネコ). And so with sake. So, I don't mind learning two whole new alphabet in Hiragana and Katakana, but I just need to know if the Katakana is just used for foreign/not Japanese words or if in practice both get mixed together. Again, if there is some rule of the usage.

I hope I didn't break any rules here… I'd love to know these answers! If someone have some video to help with the pronunciation, I'd love to see it too! Thanks 🙂