How do u know what kind of rug???

I have never had an adult home to decorate before and now I am wondering how do I know what kind of rug to buy? Are there rug rules that other people know and I don’t? Should it be circular? Under the couch? Floating in the middle of the room? I want the space to look grown up but also light and fun? We rent and can’t change the walls or couches so I’m wanting to make it cozier with soft furnishings and a rug feels like the next step. I’m looking at some pink Persian rugs but nothing feels quite right. We like pink, green, and orange, but I’m open to all suggestions. Other ideas on what to do with the space are very welcome! Taking my time with finding the right cushions/throws to disguise the dingy landlord couches. Thank you in advance 🙏