Buying your first suit, don't pay a lot for it (<£200 all in): you don't know what you're doing. Go to somewhere like Debenhams, M&S, John Lewis (classic middle class havens). Don't get something from a place like Zara or any "young" persons shop - those suits always look too small and made from strange material with odd patterns.
When you buy a suit - just get an all purpose business suit. That means navy blue or charcoal grey. Buy a white shirt (ideally a slim/tailored fit - and a sensible collar [cutaways are in fashion nowadays] and a solid colour tie, can't go wrong with blue/red.
When you are trying on a suit remember to carry your wallet/phone is your pockets. These can have a surprising impact on how the suit sits on your body - it can particularly affect the collar of your shirt.
Most suits you can buy off the rail are too long in the arm. This is because producers know you can take some length off, and because people are less likely to notice if the suit is too long in the arm, rather than too short. So keep in mind that you can, and often should, have the arms adjusted at a tailor (it is a cheap and easy job, but it makes quite a difference - be careful buying suits with buttons at the bottom of the arm, it makes them harder and more expensive to adjust).
If you find a suit you like, make sure you buy a second pair of trousers that go with the jacket. This will save you money, because you won't need to buy another suit. It will also mean you need to dry clean your suit less often (the jacket rarely needs dry cleaning, but the trousers do).
Also, if you end up buying a suit, a shirt, a tie, extra trousers, don't hesitate to negotiate a discount.