Welcome,
It's tempting to ask how you got the position if you have no idea what's involved in an external audit.
HAve you not experienced an external audit before? You need to be fully familiar with the relevant Standards against which you are being audited, and with your organisation's procedures and work instructions, and the records generated. You need to make sure that all relevant staff are briefed on how to conduct themselves during an external audit. You need to ensure that you have all the records required by the standards and your own procedures - are your internal audits up-to-date, management reviews held and documented, corrective actions closed etc.
You also need to refer back to the previous external audits on your current audit cycle and check that the nonconformances and observations raised have been addressed, (even if, in the case of an observation, that you've decided not to do anything yet....)
Don't be afraid to come back and ask more questions here, there's already some good advice above; the external audit, unless it's a full recertification audit, should only look across part of your operations and the scedule/program that the auditor has to provide you with in advance will give you a guide as to which areas will be audited.
Good luck !