You'll likely blow the first couple of interviews, Ganda...that happens a lot. It's nothing to worry about if you learn from it though. Pay attention to the kind of questions being asked so you are better prepared nextr time.
Keep in mind that the interviewer is likely as nervous as you are...most people aren't full-time human resources people.
Speak clearly and don't be evasive.
Take notes, ask questions.
If possible, find out something about the company and the position you are being interviewed for before the interview.
Dress appropriately. If you're applying for an office job, wear the kind of clothes workers there wear, if you're applying for a construction job then dress like a construction worker.
Stress your strengths. If you're good at something, don't be afraid to say so.
Admit your weaknesses. If you suck at something and tell them you are a pro at it, you won't last long.
Be specific. If they ask about computer skills, name programs you can use. If they ask about framing a house, talk about l-studs.