For now, it is something that you have to purposefully target. There are a few resources you can use to help in improving your grammar and spelling. If you are really intent on speeding up the process , go to your local college and enroll in a grammar refresher course often centered towards businessmen. This is a little pricey, though, compared to studying alone. Strunk and White's Elements of Style is the classic manual of grammar, but beneficial modern resources for writers include Working with Words and When Words Collide.
Although you won't need to find out everything written down in these books, most of it will most probably be very useful. Take out your word processor or a notebook and a pen and take down all the grammar and spelling lessons that are especially helpful to you. In this manner, you will not have to sift throughout the entire book when you need some consulting because you've got your own customized list for you.
Get a mentor, too. You do not have to pay for anybody to check your grammar and spelling all of the time, though by all possible means do so as long as you can afford it. A tutor could be a friend with a good handle on the English grammar, one of your parents or a former English teacher who is keen to provide assistance. A lesson doesn't need to be an official, classroom-type lesson too. Simply having your tutor gradually correct you while you talk or write to each other is already particularly helpful.
And, last but not the least, read, read and read some more. Reading the bestsellers, the classics and prize winning pieces of written work not only gives you the best ideas for your essay, it also exposes you to the world of error free writing.