In order to be successful with a newsletter, specialize in a subject not adequately covered in existing newsletters. A subject which you can give more, or better information on.
To determine what to write about, first look at your ability, experience and interests. Then sit down at the library with the latest edition of the "Directory of Newsletters" and make a note of the publishers of subjects comparable to what you would like to write about.
Send for a free sample of each of these to review. Find out how they are put together and whether you can do as good or better with one of your own. Make an analysis of the good and weak points of the various newsletters to build ideas for your own.
Plan in advance how your layout will be set up, what kind of photos or drawings you might use, printing and distribution costs, and all the other business details necessary to launch and maintain a continuing weekly or monthly newsletter. Be sure you have time to handle all the details required to get the newsletter out on time every time! You will need to build as much of the basic information as possible well in advance, perhaps 15 to 20 issues ahead, then feed in the current information between each issue.
You cannot hope to survive against the competition of the large firms who have many different contributing authors, and access to instant worldwide information through computer data centers, etc. So stay away from subjects put out by them unless you can develop a very unique theme to wrap around their information. Most of these firms put out a newsletter without any revenue from advertisements or inserts. Generally the little publishers must develop their newsletters in such a way as to include income from advertisers in order to return a reasonable profit and stay in business.
Research the market to determine what your "category" of subscribers want to hear in your newsletter. Give it a "stand out" title, price it fairly for both the subscriber and yourself, keep the graphics and printing clean and neat. Develop your own individual style in your writings to give your newsletter its own distinctive "personality".
How would you present your information to an individual in a personal face to face conversation? Use the same presentation in writing. It will take time to develop an individual writing style, but as the saying goes, "practice makes perfect"! So don't be discouraged if your first few attempts appear ridiculous to you. A you practice you gain experience and before you know it, you can put out information your subscribers can enjoy and benefit from.