'How to' is really important. You must start on the front foot and going in to meet someone who could end up our boss and giving you the chance at a great company looking shoddy is unforgivable. To yourself.
Best option is to ask a professional. There are many high end shops that can dress you well and give you that cutting edge but steer clear of fashion (unless that is the area you are interviewing for). Classic dress is essential for all areas in the job market. And remember this. A suit is rarely seen as a reason to not give someone a job just because they may have overdressed a little. Turning up in trainers when inappropriate is though.
Suit - if you can afford get a tailored suit. Plan ahead. Just because you may be in a safe job now doesn't mean it will always be the case. Get a well tailored suit for that occasion when you need it. Also it will be available for those things like weddings, funerals, receptions and so on. But first, it's for the interview.
Must be dark navy or charcoal plain or slight pattern at most like a herringbone or hopsack. Please avoid black unless you have to. It never looks that good except on tanned men (never if you're pale) and shows up every piece of fluff and lint you've picked up on the bus, tube or train there.
The suit style should be classic and not trendy. No velvet trim please or über-skinny lapels. And get a suit that fits you in the trouser leg length. Having ankle swingers drain pipes, no matter how in fashion, is a quick way to making you look like a boy rather than a man.