Interview

You mean being fired? That hurt! Sorry to hear that. This is some advice you can refer to:
In order to get a new job after being fired, you need to prepare not only on Interview but also Resumes and Cover Letters. Remember there is no point in bringing up the circumstances of your leaving until you have to in the Resumes, cover letters, and job application.
Interview: Here's where getting fired is going to matter most. You can be sure you are going to be asked the question "Why did you leave your last job?" I recommend you should volunteering that you were fired even before the question is asked, then moving on and telling the interviewer you learned a lesson and explain how you benefited from the experience. Take the negative and turn it into a positive.
To prepare for the interview, you need:
Practice. Take some time to prepare answers to questions about being fired so you know exactly how you are going to answer. Practice again, so you can respond confidently and without hesitation. The more you say it, the less painful it will be.
Don't lie. Most companies check references and check background information, so if you lie you are probably going to get caught.
Do not contradict yourself. Tell the truth and have one story and stick to it regardless of how many people are interviewing you. They will compare notes afterwards and you don't want to have told one person one thing and someone else another story.
Do not insult your former boss or your former employer. No employer likes to wonder if you will talking about them that way in the future. Also, don't be angry. Feeling angry after being fired is normal. However, you need to leave that anger at home and not bring it to the interview with you.
Keep on Moving:
As hard as it may be, and it is hard, you need to get over getting fired and move on. You need to be able to convince employers that, regardless of what happened in the past, you are a strong candidate for the position and can do the job. Focusing on the skills and experience you have, rather than the firing, will help sell you to the employer and will help you get the job.

@ Trung: In this case (or anycase), I always suggest to be very honest. Tell the hiring manager that you are now not working at any company and looking for suitable opportunity.

Instead talking bad about old boss (for whatever reason), may be it's better to humbly confess that may be you don't fit with that job and for many reason your old boss conflict with you. Share some story objectively without adding your personal emotion .. What matter most is what is your real value and how you can contribute to this new company, then the hiring manager can still take this positively. Good luck