Honestly not every item for a baby do you need. Just really depends on what you think you might need for your baby. But for the most import things you would need crib/bassinet, car seat, diaper bag, baby tub, sheets for crib, bouncer/rocker, pacifiers(if you plan to use them), bottles, wipes diapers, a few 0-3(they tend to outgrow fast) lots of 3-6.
As for when to buy diapers I would start buying diapers. I would probably buy about 4 packs of newborn diapers as they tend to out grow. And then buy size 1 diapers. The more you have the better as they go through diapers like crazy.
I put a few big ticket items on my registry (stroller, car seat, etc) because you never know what people might get you. Or maybe a few people want to all go in on something for you. If you don't get it at your shower just buy it afterwards. I don't suggest doing that with the glider though because those can take a while to get from the store and you want to make sure it is there in time for baby.
Every family is different when it comes to neccesities for their babies. My friend doesn't have a rocking chair but I cannot imagine not having one.
A lot of the baby products just depend on where you live and your personal expectations. I live close to family so while I love the idea of a pack n play, we really won't be needing one. I also expect to breastfeed so those things I registered for, didn't buy a ton of bottles or a bottle warmer.
When you make an online registry it should have categories like bath time, clothing, nursery etc. If you take a look at each category and pick a few items you should be fine. My sister wrote a note on her registry items like we really want this, or the brand doesn't matter but we want a swing that vibrates. If you don't mind others buying big ticket items for you, choose one and add a note saying it'd be nice for people to collaborate on it.
I was on bed rest for my first and spent HOURS on YouTube watching videos of mums talking about what they loved and stuff they bought and never used.
I would put big ticket items on the registry (if you find exactly what you want of those items) b/c you'll probably get gift cards and some places like target give you a discount on items left on your registry after your due date.
You can put diapers on the registry too.