If you don’t want to make your own background image I recommend visiting www.cgtextures.com. There you can find a large variety of textures and other cool images. For this little tutorial I choose an image from the “Brick Rounded” collection. I crop my image to a square size of 400x400px. Download my image. 
Original image
Then I go to Filter / Other / Offset and enter half the width and half the height of the image. In my case horizontal: 200px and vertical: 200px. 
Next I choose the Clone stamp tool 
(just press “s” or locate it in your toolbox). Choose a brush without any sharp edges and make the size of it around 45px. Then hold down the Alt key and click somewhere in the image except on the edges made by the offset action. Next you have to draw horizontal from left to right, along the edges made by the offset action, and then vertical from bottom to top. 
Using the clone stamp tool
If you made everything correct you should have the seamless image looking something like this 
To see how it looks in action please have a look at this page.
What to do if I do not have Photoshop?
If you don’t have Photoshop you can still get hold of some pretty cool seamless background images. These are some of my personal favourite sites:
http://www.patterncooler.com
http://patterns.ava7.com
http://www.stripegenerator.com
http://www.stripemania.com