Especially as a non-expert still learning the language, how should I tag certain questions when I have no idea what the correct terminology for the topic is? I often even do not know what an appropriate English grammar tag name would be.
Take a look at the list of tags on JLU - https://japanese.stackexchange.com/tags. Almost all of the most popular tags have a description so you can know what it's for.
Also, there are a lot of straight forward tags, like katakana if you're asking about something related to katakana (I know it's your favorite!).
You are only required to use one tag when you submit a question. You should be able to find at least one straight forward tag for your question. And, like @Flaw said in the comments, there are users who will add tags for you. I know this happened to me a lot when I first joined JLU. Maybe try to watch your questions to see if this happens and learn from the tags that were added.
If you need further help with understanding a tag, you could always ask in meta or in Chat.