First and foremost, make sure that you read your assignment's instructions carefully. Your instructor may have guidelines for how long your abstract should be, how it should be formatted or what information it should include.
Abstract writing varies a little from discipline to discipline -- an abstract for a scientific report will be different from an abstract for a history research paper, for example. SAGE Research Methods' Project Planner has a nice Writing Up section that will link you to abstract tips in different disciplines: look for How do I write my abstract?
Yourstyle guideis another good place to look for abstract-writing tips. See, for example: