You may use hard copy correspondence or e-mail for sending your cover letter. Both formats are acceptable. The only difference between them is that your signature block (address, etc.) goes at the top of the page on hard copy while it goes below your name in e-mail. A handwritten signature on hard copy is obligatory.

Preparing a resume always attach a cover letter, which should contain an explanation why you are sending a resume. A recipient will see the cover letter first and therefore, it must be very well written and targeted to that employer. Depending on it an employer will take a decision.

Generally cover letters is divided into two types: a letter of application and a letter of inquiry.

Letter of Application
In this letters applicants ask for an advertised opening.  You can say something like "I look forward to hearing from you." If the employer hasn't prohibited calling and you have phone number, you can take the initiative to follow-up, saying something like, "I will contact you in the next two weeks to see if you require any additional information regarding my qualifications."

Letter of Inquiry
This type of cover letters implies asking about the possibility of an opening. But in any case you shouldn’t wait the employer will contact you. Say something like, "I will contact you in two weeks to learn more about upcoming employment opportunities with (name of organization)."  Then don’t forget to make the call at the appointed day.

Information-Seeking Letters and Follow-Up
You’ll not be able to produce an effective cover letter without indicating that you know something about the employing organization. Sometimes, even research efforts come to little and you don’t have enough information. A good way out this situation is to write requesting information.
If you’ve got the desired information you can write a follow-up letter that thanks the sender for the information; promote why you would be a good job candidate for that organization based on the information; and explain why you are sending your resume.