Comparing two means

Hi. I have some trouble comparing two means in SPSS.

I am writing a thesis about audit exemption in Norway, and have found 48 000 companies in the population.
11 000 of these are registrated with e - mail, and are used in my sample.

To defend this sample I have to compare the mean of income in my sample, against the population, and conclude that there isnt any difference.

Witch test should I use?
I have tried the "paired sample t - test" but this isnt working if the sample sizes differs?

Answers are appreciated! :D