I believe that the best way to improve grammar and sentence structure is to read A LOT of English texts, which will of course contain those grammar rules in them. Once you know what a sentence should look like and what words go together, you will be more confident in writing your own correct sentences. Teacher supervision is also very important! There's no point learning something if you're going to learn it wrong :)

Grammar In Use by Murphy is excellent for study, practice, and review. Frequent (easy) writing with corrections by a teacher is a powerful way to address your mistakes (as opposed to trying to learn everything).