Do not start writing the resume until you have any idea about the employer. Also, you should remember this important point: you need to show the employers what you can benefit them but not what you may benefit from them. The perfect resume must focus on the strength in necessary experience and skills that the employer may require from you. You will score more point with your knowledge about the employers and understanding of what they expect from you.
The employers are very busy as they may receive hundreds of resumes and resumes each day from different candidates. So, think carefully before you send your resume and resume to an employer is a good way to win over other candidates.