That depends on your own superstitions. I'd say most people register/buy things in the 2nd and 3rd trimester mostly because the risk of miscarriage goes down considerably. But if it doesn't bother you, there's no reason you can't start now.
DD1: June '16 DD2: March ‘19 :::: Married since 2011 :::: USN Wife ::::
I bought a car seat when I was 10 weeks this time. The kind I wanted was super on sale on Amazon. I knew I could return it if necessary. It's really what you're comfortable with.
Re: When is it a good time to shop for baby?