After our gender scan we went and bought 1 thing, then around 23 weeks we slowly started buying things. Am glad we didn't wait till the last minute because we were able to be picky on what we wanted for his room, and only bought what was perfect rather then whatever cuz we were close to d-day.
Also only bought decorations and the crib, we waited till after others bought things, like dad, grandma, aunts, because they wanted to get us some of the big ticket items, like car seat, stroller, ect. So i'd check with everyone before you go buying to much or they might be mad that you didn't save anything for them to get.