This is for you first year teachers who won't have the money or resources yet when you begin your new career. BEG, BORROW, STEAL! Begin by going to experienced teachers in your district and asking them for assistance and ideas to begin your classrooms. Borrow books from the library to have extra reading materials. Also, I got a lot of mine from garage sales. Don't spend a lot of money you don't have. Make copies of materials that you can. Don't worry about bulletin boards, most districts these days want Focus walls now.
Put out the word in your district that if a teacher is throwing ANYTHING away, you will come and get it. Even if it is not your grade level. Then you can start to trade with it. You don't need a completely covered classroom. Let the students fill it up. Have them bring things for the walls. These are the most interesting walls anyway.
I used to belong to a bulletin board club and got pre-made boards for about 2 years. I finally gave them to my niece. I just don't do bulletin boards. She uses them all the time. I would rather have the kids work up.
Steal ideas from everyone! That is part of being a teacher. Use your creative energy and make the idea your own. Most schools have dye cut machines so you can make your letters and such. You can also make banners and things on the computer. (Aren't computers wonderful?)
Some districts give first year teachers an amount of money to begin their classrooms. Check into that before you spend out of pocket.
Finally, join a teacher web site like edhelper.com . It doesn't cost that much and you can make all kinds of worksheets and activities on them. Plus, you can use what others have made too. These sites are well worth the small amount you have to pay to be a member.
Remember, I have spent a lot of money on my classroom and ended up giving a lot of it away. Be frugal and smart about what you spend.