How to write depends upon what kind of writing you want to do, for what purpose you are writing and who the recipient is. Different kinds of writing require different language and different ways of organising the text, so the first thing to do is to take a look at as many different texts as you can. In general, to improve your writing it's important to read and write as much as possible, so keep an eye out for good examples of letters, articles and so on. Using the internet to read magazines, newspapers and other text-types from online media is a good idea. If you are interested in academic writing, then our Writing for a Purpose section might be what you are looking for; to learn how to write business emails, see our English for Emails section.
If possible, you should get feedback from a teacher or knowledgeable friend on your writing. Unfortunately, we don't have the resources to give users individualised feedback on their writing, but you can still use LearnEnglish to do some work on your own. For one thing, you can respond to other users in the comment sections to carry on a written conversation. Good writers learn from reading other writers' texts, so you could learn a lot about writing from reading the content on the site. Our Magazine might be a good place to start if this interests you.
Whatever you do, try to spend at least 15 minutes several days per week reading and/or writing. When you have doubts, ask a friend or teacher, or you're also welcome to ask us periodically here by using the comments sections on each page. Remember also that it's important to write in an organised way: start by collecting your thoughts, then plan how you are going to organise them, then write a first draft. After that, check (or get someone else to check) your draft before writing your final version. Research shows that good writers constantly review their work and amend it, so this is a good model.