Need help with hard drive!

Hello,
My problem is that I upgraded my Win98 OS to Windows 2000 Professional. The problem is that I partitioned my hard drive wrong and can’t fix it! When I check my hard drive in Windows 2000 it has only 800 MB of space which I put in wrong, when my hard drive has 15 GB of space. How can I get the rest of my hard drive space in Windows 2000. Repartition, delete partition? don’t know how Please help! Anybody. Thank you