Here is an area where most of your studying can be done independently.
Try some of the quizzes on this website. http://www.roadtogrammar.com/
This page also is excellent.
Let me know what areas of grammar are difficult for you based on this.
It is important to know your verbs as they are at the centre of every clause. You must know the spelling and forms of both regular and irregular verbs. You must know their infinitive, simple past and past participle forms. Study the irregular verbs here and test your ability here and here. Remember for regular verbs to double the consonant following a short vowel (plan –> planned) but not for a long vowel (float –> floated).
You must build up from words (you must learn vocabulary and spelling), to phrases, to clauses, to sentences. Then you can start writing paragraphs and essays. Here you will need ideas (both quantity and quality) and use your grammar and vocabulary to organize and write them down.