Once you have written and organized your information according to the type of resume you have chosen, be sure to format it according to typical professional standards. You should use consistent spacing throughout, and evenly sized margins on all sides if possible. It’s generally best to stick to your word processor’s default settings, but in some cases, if you shrink the margins on the left, right, top and bottom, this can help buy more space to fit your resume on one page.
However, this is a case where you’ll want to use your common sense. If you went to college for marketing and had a marketing internship your senior year, then worked as a server for the next several years, you would want to include your marketing internship.