How do you say "I do/I'm doing something great"

[Kerskjee]

So I’ve just learned a bunch of things surrounding “big” (大), and the one in question being “great” or “big deal” (大した). I’ve also learned that “to do” is “する” and “did” or “have done” is “した”.

So the way I see it is that you are “great” or a “big deal” because you “did” or “have done” something great or important, but if I “do” great things or I’m “doing” something important it doesn’t seem like you’d say “大する” (at least I didn’t find it).

So what’s happening here, am I looking at it wrong, and how would you say “(I) do great (things)” or “(I’m) doing (something) important” in a similar/simple form?

PS: I do understand that this is above my level and I’m not looking to nail the grammar or anything like that any time soon. I just want to get a lay of the land so to speak, and get a feel for how to look at or think about the language. Being Norwegian it seems Japanese deviates a bit more than English from my native language in terms of how to look at or think about it.

[Leebo]

“great things” or “something great” would be 大したこと

Though, usually you hear this in something like 大したことじゃない (It’s not a big deal).

You could say something like (私は)大したことをする to literally mean “I do great things” but… well, I’m not going to tell you what you should or shouldn’t say since I imagine if you have a reason for saying it you probably already know how it’s going to sound.

“important things” would be something else. 大事なこと or 大切なこと, for instance.

@Kerskjee

So the way I see it is that you are “great” or a “big deal” because you “did” or “have done” something great or important, but if I “do” great things or I’m “doing” something important it doesn’t seem like you’d say “大する” (at least I didn’t find it).

大した is a pre-noun adjectival. It can only directly modify a noun. And it’s etymologically related to する, but it’s not something that can be converted.