confidence level and confidence interval?

Hi,

I am currently writing my dissertation and I am in the process of analysing the data from my survery, I have been told I need to consider confidence level and confidence interval.

I have done some research and I think I understand in basic terms what they are but I have been searching everywhere but I cannot find out how to work them out or apply them. I found this confidence interval calulator:

Sample Size Calculator - Confidence Level, Confidence Interval, Sample Size, Population Size, Relevant Population - Creative Research Systems

Which is fine if i assume a 95% confidence level, i guess this is where i am getting stuck, what is my scientific justification for using a 95% confidence level?

I am a bit stuck on the whole thing so any guidance would be greatly appreciated right now...

Thanks,

Richard