Practice is the only way to learn. Try posting your English writing in the notebook section of italki.

Follow good, proper examples. These are easy to find. Don't worry about the grammar as a beginnner. Just repeat the right phrases.

I would not focus purely on grammatical rules. English is not a language that is defined by grammar, it is described by grammar. Many native speakers do not even know, or learn grammar, they simply know what sounds right. There are also things that are not even described by grammar, but yet that we know, and would recognise as wrong if you didn't say or write them correctly. Learning what is right is what you will learn by practice, as Jeff suggested.