Swap partition considerations
Swap area is necessary only to supplement RAM. Simplifying a bit: When your computer is running short of Ram it uses Swap as virtual RAM.
In my experience, Ubuntu uses very little RAM when compared to other OS's. SO if you have, say 4 Gb of RAM, it is unlikely you'll ever need it unless you run programs that require a lot of RAM (photo and video editing are examples).
In any case it is good to have a Swap area just in case. 3 to 4 Gb should be enough.
How to add a swap partition
To create a Swap partition you'll need to modify your partitions using gparted. If you want to reduce the Ubuntu partition by 4gb to make space for Swap you'll have to boot from a liveDVD or live USB because you cannot modify a partition that is mounted.
How to create a swap partition depends on your current partition table. You can only have 4 primary partitions. So if you do not have an extended partition (which can contain many logical partitions) and you already have 4 primary partitions, you'll have to delete one partition, create a new extended one and then recreate two new partitions or whatever is needed.
Make sure you keep a backup of anything important on the drive, because partitioning is a low-level operation and a mistake or a power failure at the wrong moment might cause data loss.
So boot from a liveDVD or liveUSB Ubuntu, run gparted, reduce the Ubuntu partition by 4 Gb or so. create a new partition setting it as swap.
After you do this you might need to update grub by typing
sudo update-grub in a terminal before restarting your computer.
If you require more specific information, specify how your disk is partitioned.