The majority of beer festivals I've been to (and it's been a lot), you pay three or four quid entry fee and then another £2-£3 deposit for a glass. Often you get to choose between a half pint or a pint glass. You'll also get a list of beers which you can just throw away. If it's a CAMRA festival and you're a member, you normally get free entry, unless it's the GBBF.
Head up to the bar, take a look at what's on offer and pick whatever name you like the look of. I tend to start with lower strength beers and normally ask for a sample first - They'll pour a small amount into the glass for you to try. If you like it, get it topped up, if you don't, say so and they'll pour it away. Don't take advantage of this though and just use it for free booze!
I normally get the pint glass but order half pints. Most places will serve you slightly above the half pint line anyway.
Most festivals have food and quite often it's a BBQ or a hog roast. At the very least they'll have pork scratchings (essential) and crisps/nuts.
Some places have a token system where you buy a number of tokens which you can exchange for your beer. Personally I don't like this way of doing things but I can see the advantage in that the staff serving the beer don't have to handle any cash.
Enjoy it! Use the chance to sample beers you've never had before.