You will need to reformat the drive as "Mac OS Extended (Journaled)" using Disk Utility. You may also need to repartition it, depending on whether what you did with Windows changed the partition map scheme.
If it's not GUID, select the partition tab, click the Options button and change it to GUID. Then set the Partition Layout to one partition (or leave it set to "Current" if there's already only one), set that partition to be Mac OS Extended (Journaled), and click the Apply button.
As mende1 said, you hold command-R at startup to enter recovery mode, which allows you to access Disk Utility or reinstall the system. Note that an internet connection will be required, and you will need to download about 4 GB of data (or more if Windows destroyed your hidden recovery partition and the machine has to boot over the internet from a system on Apple's servers).