"It's coming together" means "it's starting to be successful" or "it's getting closer to completion".
Use this phrase to talk about a project. It can be one of many kinds of projects:
- a school assignment
- a construction project
- a website that you're building
- a novel that you're writing
You can also say "___ is starting to come together" to talk about a project that's a little further away from being finished.
This is a good general-use phrase; it's neither very formal nor very casual.