You could study a few books to learn how to write great salesletters, but if you are a beginner you might get confused in this way. It's a better idea to buy one good book and try first learning from it. Then you might get another book. But it takes time, so in the meantime you can read this article and it should be enough to write a good salesletter. To be successful at internet marketing you need to first learn a little bit, and then take action. You will learn by doing it. It's the best way to learn. So let's learn the basic steps of writing salesletters.

First, read some other salesletters about similar products as yours. Of course, don't copy or plagiarize, but you get some good ideas and get inspired.

Second, write an interesting headline. The headline is very important. If the headline doesn't catch people's interest, they won't read your salesletter, however interesting it is.
Write the headline in red, black or blue and use bigger font than for the rest of the page.

Third, tell a story. Tell about some problems people have in your niche. Then tell them how the thing you are selling can solve their problems.
Fourth, using bullet points or lists, tell people about all the benefits of your product. Write about as many benefits as you can.

Fifth, finish your newsletter calling for action. It means that you need to tell people to "buy now". You need to tell them also why they should "buy now" Try to find some good reasons why they should act now. Otherwise people will procrastinate.

You can also make your salesletter better if you use the word "you" often. Use it more often than "I", "we" or "us" combined.

Make your salesletter simple. The background should be white. Most of the text should be black. Use other colors to emphasize certain phrases.

Use a lot of "buzz phrases", in proper context, that are important to your niche. Make these words more visible by highlighting or underlining them, or make them bold, italic, etc. But don't overemphasize. Be selective. Emphasize the most important phrases only.
Don't use caps too much. Don't use too many images either. Better none, than too many.
Offer money back guarantee. It will increase your sales conversion.
To make people "act now" offer some bonuses.
Be honest; don't over-exaggerate the benefits of your product.

Don't try to make your salesletter perfect at first. Just write it, and then you can work on improving your salesletter.