If you wanted to remove your Windows partition, you should have used Boot Camp Assistant in OS X to do so.
Anyways, power down your Mac, wait a couple minutes for it to fully shut down, turn it back on, zap PRAM a few times, and then hold down the Option/Alt key. Choose your Mac's startup disk and you should be all good to go. Make sure to choose it as your permanent startup in System Preferences > Startup Disk.