Start by setting your DSLR (A Canon 600d in my case) to automatic mode. Simply press the menu button while in movie mode and it's the first option "Movie exposure". Set this to Manual and you're done. At the bottom of this menu, there's something called "Highlight tone priority". Disable this then go to the next page in the menu. Set the Movie rec. size to 1920x1080 and if you're camera is set to PAL you can choose to shoot in either 24fps or 25fps. Personally, I shoot all my footage at 25 frames per second.
Next up is exposure. You can check for correct exposure by hitting the Av +/- button and a meter will pop up. Press the * button and a little tab will appear on the meter and you need to get this tab in the middle between the 1s for correct exposure. You can adjust exposure through the ISO, aperture and shutter controls. The dial on the top of the camera controls the shutter. Ideally, you should leave this set to 50 but in certain situations you might want a fast shutter so you can capture fast motion with more detail. Slow shutter = more motion blur. Fast shutter = less motion blur.
Holding down the Av +/- button while moving the dial will adjust the aperture. A wide aperture (such as 1.8) will give you very shallow depth of field which will make it harder to focus but you get a very cinematic blurred background so your subject stands out more. A tighter aperture will provide a greater depth of field meaning more of the scene will be in focus. A tight aperture will make it easier to focus but it will require more light in the scene. You need to decide what's important for a particular shot(depth of field, motion blur etc) so plan beforehand.
The last setting which controls exposure is ISO. Press the ISO button and make sure it's not set to AUTO. The ISO on a DSLR is similar to the gain on a video camera. With a high ISO you get more exposure but it also introduces more visual noise on your footage. You shouldn't rely on high ISOs as an alternative to lighting your scene properly. On the preview LCD screen, your footage might look okay but when you play it back at full resolution on your PC, you'll see a lot of noise and this doesn't look very professional. It's generally recommended to set the ISO to a multiple of 160. On the Canon 600 however the ISO options are 100, 200, 400, 800, 1600, 3200, 6400. Personally, I wouldn't go higher than 400 and 800 would be my absolute maximum. However, for outdoor night shots you might have to bump the ISO up to 1600 and beyond. If you're shooting indoors, always light your scene properly.
Last of all, you may want to invest in some accessories to make your DSLR more practical for video work. Get a decent tripod with a quick release plate so you can quickly go from static to handheld. The Konig Kn-tripod is a decent affordable tripod which I recommend. For handheld camera work, look into buying a shoulder rig so you can get steady shots. Finally, the standard kit lens isn't that good so buy some prime lenses. The Canon 1.8 50mm lens is a good quality and affordable prime lens. Most beginners buy this lens before investing in more expensive lenses. If you're serious about film making, be prepared to spend hundreds of pounds (or dollars) on lenses.