Ask your supervisor or teacher about how he wants the paper formatted and follow any instructions he gives.
Consult a style guide, such as that of the American Chemical Society, if you are writing in a field where one is relevant. Other common style guides include those of the Modern Language Association and the American Psychological Association. All of these ask you to indent the first lines of your paragraphs five spaces.
When in doubt, just use the "Tab" key to indent. Most word processors, including Microsoft Word, are set automatically to space five times when you hit "Tab."