My advice to you is to look for an online thesaurus. A thesaurus will help you to learn new vocabulary words, which can help you write your ideas more expressively. If writing in Portuguese first helps you, I'd say do it (especially in outlines and main ideas); however, if your goal is to write more expressively, make sure that you practice writing your thoughts in English. Another tip is to avoid being too much of a perfectionist. Sometimes you just need to write down what you can and change or correct it later. If you spend too much time worrying about whether it's right or not, you might not finish what you are working on.
