Read a lot ..... If you have never used a DSLR or went from a compact camera you have a steep learning curve and all the things that are possible with a DSLR over a compact can get overwhelming
I bought a magazine called NPhoto and did buy the Dummies Guide to D3100 but being honest found it too simple, but these two books are a good read and have plenty of actual photo's to show what does what and why
David Busch's Nikon D3100 Guide To Digital SLR Photography
Nikon D3100 From Snapshots to Great Shots
As you are in UK you can pick it up from any WHSmith or your local newsagent should be able to order copies for you, and you can get 'some' back issues. You can also 'buy' an electronic version that is available for all issues back to issue one
Well worth a read and will help to start to understand your camera, I received a D3100 for Christmas and I am slowly starting to understand what it is capable of from a self-teach basis
Try to avoid the 'auto' mode if you can, it will help you to understand how to use your camera and make you appreciate what does what. I use aperture and shutter mode a lot, but do use manual mode when using the tripod and lens filters
If you have just started, buy a UV filter at least and leave it on your lens. If something happens at least only the UV filter gets damaged or scratched or wet rather than your expensive lens
Variable ND filters are good if you take photos that are flagged as too bright, but will then need a tripod and I bought an external shutter release cable to avoid any vibration as it will need longer exposures
I've had a quick handle of a D7000 and found it instantly easy to use after using the D3100 but didn't have time to have a real look at all the settings and dials, so had to use the 'auto' mode to take a couple of photo's at a wedding somebody asked me to do for them at short notice
Well worth a look, but out of my pocket for quite a while yet, but does mean I can still use the lenses I bought
Also buy a few memory cards, an 8GB microsd is very cheap with an adaptor. Even a class 10 8GB can be had for £5 each. I use RAW mode after looking at various forums and advice so you have a much better chance of editing it, plus JPG may be faster but is a lossy compression so every time its saves the image you WILL lose more details, where RAW is just what the sensor 'sees' so you get everything from that shot but is much bigger so hence the 8GB cards at least
I have a couple of 8GB cards and a 16GB card