What is the process for buying a PAYG phone in store?

I'm 17 and I'm planning to go into the Vodafone store to buy a new PAYG phone this weekend. I've never done anything like this before, so could somebody tell me the process of it all? Like is it complex or basic? Do they need ID or anything? Can I pay in hand cash instead of by card? I just need to know before I go in and make a possible dumba$$ of myself lol:)