Hi - Do you mean to make a good impression in the letter you're sending?
You may tell them how you came to know the company, why you wish to work there, what of your qualities you think meet their needs etc. In other words, do you homework and know about the company and in writing, be clear and sincere - check your letter carefully so you don't have a bad impression with many mistakes. In the end, you may say " I am looking forward to the opportunity of an interview" etc.
hope this will help.