The interview process is designed to find the best person to do the job, but it is also about building trust and rapport. Would they like to work with you?
View the interview as proactive, rather than passive, exercise. You have won your right to be there! Take advantage of the opportunity to sell yourself - it is not a time for false modesty, though be aware of the difference between confidence and arrogance.
Performing well at interviews is a learned process; you will get more confident with practise.
The person who gets the job is not always the best candidate on paper - sometimes this is the result of a good interview.
Leave salary negotiation until you are offered the job - there will be little or no negotiation around salary for an entry level or graduate position.
You are not alone; most people find interviews awkward and stressful. They are rare events that require you to sell yourself in a potentially competitive environment.
Preparation is the key to enable you to provide specific examples to reinforce claims about skills and competencies.
The work you have done in your application has given you good preparation for the interview. Re-read your application.
Research the company or organisation before the interview - at the very least you should know the company's core business and be familiar with its big projects and campaigns. The more you can relate your skills to the work the company actually does, the better.