As you already know, it is not easy trying to get an interview for a job position. Even when you fill out an application, you can't really say much to persuade the employer to give you an interview. With a cover letter, you can be more detailed and it makes you stand out a little more than just filling out an application would.
To me, the cover letter is your first interview with a company, which is why you need to take your time and put some real effort into it. Do you know how to write cover letter?
The cover letter basically compliments your resume and should include things like why you are interested in the company and also explain skills you may have that are very beneficial to the position you are interested in.
Why do you feel you are the best candidate for the position and you should be able to show your knowledge and experience in the field for which you are applying?
A cover letter should not be too long otherwise your resume and application will probably be ignored. No one is experienced in how to write cover letter. It takes practice and hard work but once you get the hang of it, you will pretty much be able to do it on your own with no problem.
The best thing about writing a cover letter is that you have tools available to help you such as the internet and the library.
The employment agency in your county may also have workshops to help you learn how to write them on your own and they are free. It sure beats wasting money to pay someone else to do it.
Even if you know how to write cover letter, it may still be a good idea to get some new tips. You can never know too much, especially when trying to impress an employer.