I've attached a checklist at the bottom of this page to help you think of everything you might need to purchase or do before you start the beginning of the school year.
One tip I can give you is before you buy anything, take an inventory of everything in your classroom.
One year I was unable to find out what was in my classroom before I placed my order, and I ended up ordering some things I really wouldn't have needed. Use the attached checklist to help get you started on things you will need for your classroom. You can check the items you already have off on the checklist to help you know what items you will need to purchase, and what items you already have. You may not want or need all of the items on the checklist, but they are all things I have found very useful in my classroom. Doing this important step will save you time and money!
Also, ASK. Ask your school's Main Office if they give certain supplies to teachers. My school's main office always gave us a bag of supplies each year such as: whiteboard markers and erasers, paper clips, sticky notes, etc. Our school nurse always supplied us with a box of Band-Aids and some rubber gloves for small cuts and scrapes so that we didn't have to send our kids to her all of the time for minor things.
Don't be afraid to ask! Sometimes it's intimidating as a new teacher to ask these things, but know that asking will save you money and time. Your school is there to help you! You can do it! Just ask! :)
UPDATE (7-15-12) The Clutter Free Classroom has a checklist of things to buy for your classroom as well. You can download it for free by clicking here.