Welcome Sergeantelmo, to the game and the forums!
Much good advice has already been provided, I will confirm that MANY groups (which includes spires, halls, houses, families, cartels, chapters, stridings, warbands and more) will happily accept new players in.
There are also some introduction meetings on the Friday afternoons, the Egregores (magical national spirits) are available to ask questions in play, and the phrase “I’m new to Anvil” is a useful in-game one that will cause people to explain carefully
As for nation, magic is the Urizeni “hat”, but you could pick and choose really.
Costume: note that the people in the photos and the costume guides may have spent years building up those costumes a bit at a time! It’s intimidating, but that’s an end-goal more than a minimum standard.
There will be traders at the events, running from Das Shop (which is cheap, and accepts swaps) up to the high-end stuff where a shirt could cost you £50. For a frame of reference, there’s a list of traders on the wiki…
…and as mentioned, a surprising amount of kit can be cheaply assembled. Charity shops, cheap&cheerful outlets like Primark, and vintage/costume shops may have a variety of things you could use or adapt.
For example: A pair of plain cotton trousers, green or brown or black, no logos and not too new. A simple shirt, off-white, with a couple of buttons at the neck. Possibly £5 the pair from a charity shop. Adapt with scarves, belts, maybe an old waistcoat from the same places… Perfectly reasonable and basic “soft kit” (hard kit tends to be armour and top layers).
And that’s without you hunting elsewhere and being willing/able to do some tinkering. Ebay probably has a lot of old costume (from LARP, historical re-enactment and theatre) on it. Minor sewing (ribbons, patches, buttons) or painting (thorn designs, stylised animals, heraldry) can make a lot of difference.
Some basic kit + one or two other bits (tabard, cloak, bracers, sash, that sort of thing) is an excellent start.
Oh, and footwear. The general feeling at Empire is that costume stops at the ankle. Because it can be quite, quite muddy. So hiking/walking boots are often in evidence, as are cunningly disguised wellies In the summer you’ll see people in impressive boots, sandals, bare feet… but don’t start like that…
The rule is that costume is aspirational. So turn up in the best you can manage, and people will be happy to give hints and suggestions as to upgrades.
And as with a lot of things, whatever you wear, someone will look much worse than you, and someone will look much better.