Most certainly do not wear the piercings.
I doubt you'll need your certificates unless they ask for them beforehand. They'll ask stuff like why do you want to work there, plans for the future, what experience you have etc. Never answer that you're only doing the job just for the money.
Look smart, keep calm, answer honestly and above all be punctual.
As you've worked in a store before, you can say that you have till experience. With Costa it'll be important that you can follow instructions to make the coffees perfectly - saying that you understand they're very high quality drinks that must be made exactly would be a good thing to throw in. Do some research on Costa as a company; stuff like when they were set up, what charities they support (hint, they run a very successful one in Africa that you should know about), where they first started, where they source their coffee etc. They won't expect you to be absolutely perfect on it all but showing that you've researched the company and job is a really good thing as it makes you look really interested.
They'll probably ask you why you're interested in the job, whether you'd progress it, what your best qualities are, why you think you're best for the job.
Great them with a hand shake, introduce yourself, be really friendly and above all honest. There's no point saying that you're really shy and quiet if actually you're really loud and out going because they'll find out when you start working there