Click on the magnifying glass in the top right of your screen.
Enter "Disk Utility" in the textbox and hit return.
When it opens, select the entry in the top of the left hand column. This is your main hard drive.
You should then see "First Aid", "Erase", "Partition", "RAID", "Restore", select "Partition" in the right panel.
See this link for a visual guide.
http://www.creativetechs.com/iq/leopard_disk_utility_can_resize_partitions.html
You should now see two partitions in the graphical view. Select the lower one (Boot camp) and hit the (-) icon below it. Then, follow the instructions in the linked guide to resize the partition.
B