I have a Canon 600d. I need to have a long exposure. What settings should be used to achieve a long exposure in my camera? I tried searching and most of them say bulb mode. I don't see it in my camera. Any suggestions on how to achieve long exposure?
Take a look at page 100 of the manual. It explains how to use Bulb mode.
Put the camera into Manual(M) mode using the top dial, then turn the dial to the left to select BULB.
You can also use an intervalometer to capture long exposures of varying times more accurately then in the bulb mode.
Get Magic Lantern.
Opens up a whole load of neat stuff including an intervalometer. And it's free!
In addition to what dpollitt wrote about using Bulb mode, you may want to consider buying a remote control/shutter release.
It looks like the Canon RS-60E3 will work with the EOS 600D, but there are aftermarket variants as well, and you should double-check compatibility to be certain. The specific model number Canon unit will almost certainly be listed in the manual under accessories.
A remote control will allow you to keep the shutter depressed, or even lock it in a "down" position, without risking vibration to the camera body, thus helping significantly with long exposure photography. This becomes even more pronounced with mirror lock-up (don't know if the 600D has that feature), as it can nearly completely eliminate camera vibration.