First time DSLR user

So I have just purchased my first DSLR: a Nikon D3100 with 18-55mm AF VR lens. I love the camera but am stuck on what to purchase next. I have £100 to spend on additional accessories. Should I buy a Nikkor 50mm f1.8 AF? I'm very interested in experimenting with shallow focus in both photo and video - but is it worth spending £90 on something that might not be too different to the kit lens? I already have a camera bag, UV filter, pop up flash diffuser, lens cleaning kit and memory cards. Should I go ahead and get the lens or should I be looking to buy something else?

I would suggest you wait until you know what you want to shoot. Put that money away until there comes a time when you need it. Wait a couple of months until you know what you shoot regularly. If it is portraits get the 50mm f/1.8.

The most common "next' lens for a beginner photographer is the complimentary 55-200mm VR lens from Nikon. It completes the basic range allowing wide angle to telephoto.

Boy oh Boy, you are bitten! That (L)100 is burning a hole in your pocket. Wait until you find an area of photography that can be solved by buying a specific lens. Not buy the lens and hope for that "area" to manifest it self. In all probability you will need more money. So save, save. And then get a really nice lens that will do the job for which you have a need for.

You can experiment with shallow focus with any lens. Before the digital world came up with their "perfect portrait" lens, we (photographers who did this for a living) used either 85mm for young faces in portrait, or 135mm for all general portraiture. (85mm & 135mm are film format lenses, you will want the digital eqvalant, say a 50mm).

While a 50mm f1.4 lens is definitely a plus in low light, especially if you want to handhold your camera, you may want to consider a 50mm f2.0 or a 24mm f2.8. Prime lenses faster than f 2.8 tend to be pricey, but justify their cost over the long run by allowing you that one shot you couldn't get otherwise. But, since most digital cameras now have image stabilization, it's much less of an issue than it used to be. You can now use your camera handheld in low light conditions that used to be impossible without the use of a tripod. Really, the systems are that good. If you really want to experiment with shallow focus, try using a medium telephoto at around f 2.8. You'll quickly see that your hyperfocal distance - the area of the photo in front of and behind the subject that appears to be in sharp focus - is quite limited. A 105mm f2.8 medium telephoto is generally considered the perfect lens for portraiture, not only for the natural perspective it lends to the human face, but also for its shallow depth of focus at larger (lower numbered) apertures.

It seems you already have realized that your kit lens isn't good enough for what you want to do so go ahead, get the 50mm f/1.8. Yes, blurring the background would be easy with f/1.8. I highly recommend you keep the kit lens for more general purpose shooting.

If you buy the 50mm f1.8 it will not auto-focus on your D3100.

Save it! Work out what are the limitations of your current setup, and what specialisms you wish to explore before you start throwing money at your hobby.

Learn to take full control of your camera, then you can explore what it's really capable of.

I'll tell you the 50mm f1.8 isn't the lens to get though, it's doesn't autofocus on the entry level Nikon cameras. You will have to spend more on the AF-S 50mm f1.4, but for now don't worry about it, think more about it three to six months time when you can better evaluate where you are with your photography.