Get books and study guides on English grammar. When you finish studying specific chapters or lessons in the books, complete any practice exercises in some of the books so you can get a better idea of how to spell common English words and write grammatically correct sentences.
Meet with a tutor on campus if you're a college student. For example, if you need specific assistance on forming sentences which have proper subject-verb agreement, tell him or her your struggles in this area and request to meet with the tutor a few days out of the week to improve your sentence formation skills. During the sessions, take good notes and ask questions on anything the tutor teaches that you're unclear about.
If you're good at forming sentences correctly but not good at spelling, get an inexpensive pocket dictionary and look up the words you misspell often. Then write these words down on a sheet of paper and practice by using these words in short paragraphs. Also get a relative or friend to quiz you on the spelling of these words so you can figure out which words you still need to practice with.