What exactly to buy next for dungeons and dragons?

Ok, so recently for my birthday my friend bought me the Dungeons and Dragons 4th edition starter kit. I have a group of 5 of us who are all willing to put time and money down to get into this game. (All of us are new to d&d but have nerd experience) So basically I have some monster and character tokens, 3 sheets of dungeon tiles, 4th edition quick start rules guide, dungeon master's book, and 6 dice. Now for one thing I definitely would like a guide for making our own characters, because once we get a little more experience and finish the 3 encounters made for us already, I want to start making up our own encounters/pcs. So in your opinion, what would be a few good book buys for me to get? Also, for now I am the dm, but eventually I think it would be fair to everyone if we switched off the position.