How to do long exposure? Photography?

I have a nikon d3100. I need around 5 mins.

Set the camera for B (bulb) mode and the shutter will stay open as long as you hold down the shutter. You'll definitely need to put the D3100 on a tripod, and I'd suggest remote release cable so you won't cause the camera body to move while you do that.
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If you use Bulb mode (manual mode, set the shutter to bulb), then you can have an exposure up to 30 minutes.

In bulb mode, as long as you depress the shutter release, the shutter will remain open. However, this requires you to hold the shutter release down for 5 minutes, pretty uncomfortable, not to mention inducing camera shake by holding the shutter down.

Fortunately, there's a better way.

If you buy a ML-L3 IR remote, or a MC-DC2 shutter release cable, you can do this remotely.

If you use the ML-L3 remote, you have to setup the shutter release mode to IR remote. When you do that, the display changes from "Bulb" to "Time". The first time you depress the IR remote, the shutter opens. The second time you depress the remote, it closes (or if 30 minutes elapse, the shutter closed automatically to keep the sensor from overheating).

If you use the wired remote, you don't have to change the shutter release mode, so the display will indicate "bulb. This remote will keep the shutter open as long as you depress the button. But most of these type remotes can be locked in the "shutter open" position.

Note that the D3100 has the ability to use the MC-DC2 wired remote, but the D3000 has no connector for the cable, and can only use the ML-L3 IR wireless remote.

http://www.althephoto.com

Get one of these
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Set your camera to manual and speed to bulb. (You probably knew that part.)
Then set your shooting mode to work with the remote.
Point your ML-L3 Wireless Remote at the camera's sensor and push the button. That should tell the shutter to open. Pointing the remote at the sensor and pushing the button again should tell the shutter to close.
If you don't tell the shutter to close it will do so by itself in 30 minutes.
If the Long Exposure Noise Reduction is turned on (by default it usually is), the camera will run a cycle to subtract out hot pixel noise. Exposures greater than about 5 minutes may still show some anyway. By 30 minutes it may look like confetti.
These long exposures drain batteries.
Getting your lens to focus correctly in such low light may involve a few tricks.