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.
Do not expect to win the employer when your resume and resume are common without any particularity. So, do not waste time sending imperfect resume and resume to different employers because your chance of getting to the next round is not significant at all.