To a certain extent, what you write in a letter depends on where you are sending the letter. For example, a letter to an old friend can be written in a less formal manner than if you were writing a letter to a business. Here are some general rules which should cover you.
First, it's always a good idea to write your address in the top right corner, so the person receiving the letter will know where to send a reply. You can also put other contact details as necessary.
On the right-hand side, if you're writing a business letter, write the address of whoever it is you're sending the letter to.
Also on the left-hand side, write the day's date. This helps the person receiving the letter to know how up-to-date the information is contained in your letter.
After 'Dear Sir/Madam' or 'Dear (friend/relative's name), you starting writing the main part of the letter, where you express what you need to get across to the other person.
The Main Attraction
The first part of the letter is often just a general greeting and introduction to the rest of the letter. If you are writing to a friend for example, you could ask if the person is well, and tell them how things are with you and your own family, in terms of health and well-being.
If writing a business letter, the first part can also be kept general, perhaps referring back to a previous letter ("I write with reference to your letter dated December 1st...") or giving details of a product or service you are interested in ("I write with regards to the TV I bought from your store recently...")
Then you can get into the bulk of what you'd like to say to the other person. If you've a lot to say, then start not with the letter, but by jotting down a list of things you need to remember, as if you were writing a shopping list. As you write the letter, you can tick off the items on the list as you go.
Don't be afraid to start over again if you feel you're going wrong with the letter. The greatest writers in the world need more than one go to get it right in writing, and those people write for a living. No-one gets it right first time, every time.
One good tip is to read the letter out loud to yourself once you've finished to see if it 'flows' properly. Try to make sure the next sentence you write has some sort of connection to the previous sentence, so that you're not jumping around from one subject to the next and back again.
At the end of the letter, sign off with either 'yours sincerely/yours faithfully' for a business letter, or whatever greeting you deem fit for a friend or relative ('with love, from Tim' for example). Always sign a letter at the end, especially if writing a business letter.