You cannot reduce an active or open LVM volume without the risk of loosing data. You will need to start the system from DVD into single user mode and perform several actions. It is too complex to explain this from scratch. If you really want to know, I suggest to search for some tutorial on the Internet.
If this is in an early installation stage, I suggest to reinstall the system. Does your system have less then 4 GB of RAM? In this case you should create a swap partition that is at least 8 GB.
Alternatively, create and use a swapfile to extend your swap space.