Writing on the net or on the paper is very different. There are different rules to be followed, but both have some same norms to be respected.
1. Know the subject you’re writing of. Knowing the topic is surely an advantage.
You can write more easily and you can understand what it’s more important and what is not.
If you have to write about a topic you are not familiar with, first of all take your time to read up everything you can about it.
It is very important to be always curious.
2. Use simple sentences, better if short. Try to avoid complex sentences, in order to be clear and immediate. Divide the text into pharagraphs.
3. Choose the best words. Always look for synonymouses, so to avoid repeatition of the same words. Choose the most appropriate ones to the text.
4. Check the verbs. If possible, try to use always the same tense (for instant the present). Otherwise, pay attention to use correct verbs at the correct tense.
5. Check your doubts. If you have any doubt about a word, check it: how it is written, what does it mean, and so on. When you finish to edit your text, wait a few hours before reading it again.
Finally, a good advice to write a good text is to read a lot. Find your favourite authors and read everything you can. Moreover, write as lot as you can: diaries, mails, notes, thoughts and so on.