If we saw something really adorable or something on a great sale, we bought it, regardless of how far along I was. The big stuff, like nursery furniture, we didn't get until after 30 weeks, and then we we waited until after the shower for anything else, 36 weeks. We knew we were were getting the crib and stroller then, and wanted to see what everyone got us.
My first pregnancy I started once we found out the gender like 16-18 weeks (can't remember actual week)
With my current one we just waited until after my baby shower which was on the 5th of August and are now just buying some last minute items.
Try not to go to over board that's what I learned from my first time around since I seriously could have waited till I was further along also something's I thought I needed I did not even end up using and was just a waste.
My hubby bought a onsie when we first found out at 13 weeks. But we didnt start buying anything until after the baby shower at 31 weeks. So we knew what else we really needed for the baby. But its never too early to start buying anything, especially diapers and wipes.