If you need omnidirectional sound, then a dynamic microphone will suit you best - the type singers use. These are fairly cheap, and, unless you have a local music shot, Maplin is probably your best port of call in terms of the high street. However, you will likely need a converter that allows you to use a 6.35mm (1/4inch) jack on equipment with a 3.5mm socket too. Due to the size of the one piece connectors, I'd suggest going for a short cable converter (for example HQ 3.5 Stereo Jack Plug to 6.35mm Jack Socket cable 30cm | eBay)
If you want to pick up just a small angle of sound then a shotgun mic is the way to go - preferably with a boom pole too. The RØDE VideoMic is a popular choice with its ability to mount on the camera using the hotshoe, but can be used off camera with a few optional extras.