Job interviews can be extremely nerve wracking, and some people find they just seize up. A few deep breaths will usually help. Make sure you go prepared. Most companies have a website. These can give you lots of information about the company, what sort of customers they have, and what sort of mechanic work they do. E.g. they may specialize in Japanese bikes, or they might be a small outfit, that keeps a community together by giving hard driven utility vehicles a new lease of life. This in itself will give you a rough idea of what might be expected: hi spec Japanese bikes may well need electronic tuning and diagnostics, utility vehicles might benefit from somebody experienced in welding. Make sure you work out some likely questions and what you are going to say, and how you are going to prove that you have a good skill match.
First of all check out exactly where the job interview will be and how long it will take you to get there. Leave plenty of time; you do not want to be in a rush. If you get there early, just go off for a cup of coffee or walk round the block until your appointment time, do not turn up real early. Your appearance is the first thing a potential employer will see. Dress the part; make sure you look like a competent mechanic. Dirty or ripped clothes do not give a good impression, and at worst can present a hazard near machinery. Finally, do not make anything up, if you do not understand a question, say so, and ask for it to be worded in a different way.