Your first step is searching job advertisements to find a job or internship for which you are qualified (even if you won't actually be applying for it at the moment). Next, you need to composes a resume. A good sample of the format you should use can be found at Purdue OWL. To draft a resume, provide your contact information, including a business-like email address; what you desire for your employment objectives (although many say this feature is now pointless and passe); your education particulars; and your previous employment, intern and volunteer experience.
In your cover letter, you need to show a potential employ how your resume makes you a good fit for the position and the company. Thus, you should look at the company website and find out enough about the company to be able to illustrate how you would be a good fit to their strategic objectives. The key to a good cover letter is that it should not just be a summary of your own achievements but should show how those achievements specifically can be relevant to the job -- i.e. rather than "I was on my school basketball team for three years" say "In my position as an assistant team captain for my basketball team, I practiced the leadership and coaching skills you require of camp counselors BY ... " (and then give a specific example).