Honesty is the best policy
If new employer asks you the reason of being fired from previous job, always remember that honesty is the best policy so be open and brave to own the responsibility of your getting fired from previous job and share every aspect of that omission with your new boss. You may feel little awkward while doing so but it will really help you in long terms. The first good message you are going to convey is that of taking the responsibility of your miscasts. The new boss will must think to give you a new job realizing you a straightforward person.
Get your documents ready for new job
Since you need documents ready to walk confidently for interview, you must carry all important documents along. If you were fired due to the downsizing of organization you can still get the experience letter which will help you a lot, so include it in your documents. Do not disseminate the information of being fired from previous job if the new boss did not ask you the question. For this purpose take those documents of getting fired with you but never provide them until asked. Never bother on bringing false experience letters with you. It could put a huge question mark on your credibility if proven wrong.
Keep yourself calm and polite during interview
If the new boss asks harsh questions about your getting fired from previous job never lose the temperament at any stage. Keep yourself calm and polite during the interview it will help you to right the wrongs of your previous job. If your new boss asks few harsh questions then you must hold on your nerves and convey him politely that you would not repeat the same mistakes you committed during the last job.
Explain the things to the boss
Explain the things which led to your omission from job if you were fired mistakenly along with the experience you learned from it. It is always good to win the trust of your new employer in very first meeting cause it is said that first impression is the last impression.