Diapers and wipes, especially if you're planning to use disposables. Sam's club or Costco carry huge boxes of huggies, pampers, luvs, and store brand diapers and wipes (all but luvs wipes, since I'm pretty sure luvs doesn't make wipes)
You could also start checking yard sales and thrift stores for things like cribs, bassinets, bouncers, swings, high chairs, etc. It will save you a ton of money to start early!
Congratulations mama! Welcome to the mommy club lol.