Depends what you want to play, really. I'd be inclined to get a fairly cheap acoustic guitar first, to see if you like it. If yes, then buy a decent one (acoustic/electric/both). If not, then it's no big loss.

As for what type, go for a 6-string. 7- or 5-strings are a bit of a waste to be honest, as the extra range is either mostly pointless or redundant. Look for some of the following brands/models if you want a good beginner's guitar:
Acoustic:
Takamine
Taylor
Ovation
Electric:
Yamaha Pacifica
Squier Stratocaster/ Telecaster
Epiphone SG (if you can stretch to it - around £300 - it's a good quality, very playable guitar, with a great-sounding neck pickup. I've got one.

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Oh, and if you're getting an electric, prepare to pay to get a half-decent amp. I'd suggest something from the Marshall MG range. They're the best you'll get for the money by a long way. If you're getting an acoustic, then spend the cash on a better instrument.
