If they give you less than $45K, you had better accept the offer. At present, it is not easy to get a job. So a job is a job. Don't miss it.
If they ask you for your salary expectation, you should give a number in any case.
Some people say " I don't how much I should earn ...", but it is really bad. Frankly tell the people how much you expect. Of course, for the first job, don't ask too much.