After the baby shower but if you're not having one, get a few things around 5 months when you find out the sex. I sent out announcements with registry information in case my family needed guidance to what to get me lol
I'd say after baby shower but if not after ultrasound so you know the sex
Honestly...whenever you want. I did the bulk of my shopping for both kids later in pregnancy before and after baby shower but did pick up a few things during first trimester.
Whenever you want. I've picked up some really cute items I felt I wouldn't find later if I waited. just don't over do it if your having a shower. you don't want things you won't have time to use.