You should know what you are worth walking in the door. Get a good sense of the going rate for someone with your qualifications by doing some research on the Internet (see resource links below). Realize, though, that what you think you should earn and what the employer is willing to pay may be quite far apart. Factors that may drive this difference are geography (rural vs urban, northeast vs southwest), the employer’s industry and how the interviewer perceives your learning curve.
Salary is important, but you also need to look at the entire compensation package which includes health/dental/vision/life insurance, retirement plan, benefit time, etc. Generally, if the value of the benefits adds 15-17% value to your base salary, that’s a good deal. For example, if your base salary is $30,000 and the benefits added value is 16%, that means you are actually "earning" $34,800. It's true that you need take home page to meet your obligations, but sometimes what we don't have to pay for is "money in the bank", too. For example, a company may help you pay back student loans or maybe they will pay for you to take two classes a year to keep your skills up to speed. These are benefits to you, but don't cost you anything.
Make a list of all your prior accomplishments, both professionally and personally.
Learn everything you can about the negotiating process. Read books, attend seminars or take a class. The more you know about the process, the more comfortable you will be in asking for money. Knowing the market value for the skills and experience you have will also help you negotiate with confidence. That requires doing your research before you start.
Finally, even during those moments when you are not feeling confident, project confidence. Before you go into an interview, tell yourself out loud why you deserve to get what you are seeking. If you believe it, so will the interviewer. Be enthusiastic about the job, confident about the abilities you bring to it, knowledgeable about your market value and firm in seeking a fair compensation package. Doing these simple things will enable you to negotiate the best possible package.
More Helpful Resources
Glassdoor.com - Gives you an inside look at company interview questions, salaries, and reviews, for thousands of companies.
NACE Salary Calculator - The National Association of Colleges and Employers (NACE) offers the most accurate compensation data available.
LinkedIn.com Salary Tool - See a detailed breakdown of salaries by job title and location.
Salary.com - Use the Salary Wizard® to conduct research on salaries of a variety of occupations.
Evaluating Your Benefits Package - CollegeGrad.com provides a list of pointers to consider
Benefits and Compensation Glossary - from the International Foundation of Employee Benefit Plans