I’ve just skimmed the answers above but they seem to have some good advice.
To add my 2p, preparation is the key. Get a book called Brilliant Interview. The same author also has a book called ‘How to succeed in any interview’. Both books take you through the entire process from application to accepting the job.
I interviewed candidates for a role recently and I had one guy who had been in the army for 20 years and was looking for his first role outside, he didn’t answer the questions in a way that gave me any indication of how he’d approach the role despite me asking the same questions in different ways. It was apparent that the army hadn’t prepared him at all for interviews. One of my colleagues, who was also in the army, confirmed this and told me about a BT course that he’d attended, it was a free one day course on interview technique and how to approach them. I’m sorry, this is all the detail I have but a quick google will probably help you.
Good luck and try to relax, don’t put too much pressure on yourself to get the first job, interview technique is something that comes with practise.