The easiest place to start I think would be with your reading. It will provide an example of what written English looks like. Also, practice makes perfect! Try writing a few sentences to summarize what you read. Or maybe write short paragraphs about a simple topic; about your weekend plans or a favourite movie. Highlight words or phrases you are unsure of and correct and practice them. If you are looking to make your writing more interesting or exciting, you can practice expanding your vocabulary. For example, instead of a meal being "good" it can be "delicious, savory, mouth watering, delectable" and so on. If you want to improve the formalities of your writing, say for an essay or work project, you may want to focus on improving spelling and grammar.