Read, read, read and..read some more.
I was in school when the great dumbing down in the education system was still in its infancy and paid dearly for it.
I had a rough time with English and failed miserably. When I retook English in college I had a new teacher who was sublime. She worked hard on me and I did much, much better.
She recommended books to not only get used to absorbing reams of English but also to develop critical thought. I read the following several times: 1984, Animal Farm, Fahrenheit 451, The Crying of Lot 49, Catch 22, Slaughter House 5 and Catcher in the Rye.
After college I spent years reading any book that tickled my fancy and would often reread them.
I found beauty in words and sampled a newly discovered one like I would a delicious morsel. They became my food for the brain.
For the novice reader it is a chore and sometimes you can read a paragraph and not comprehend it. Don't be put off because in time, you will marvel at the depth of your vocabulary.
Don't be embarrassed but you might want to start with childrens books. Larger text and the simplified structure makes a good introduction to fine English.
E.B. White's Charlotte's Web and The Trumpet of the Swan are excellent. They are aimed at the very young but the quality of the writing is wonderful.
Don't forget that the more you read the easier it gets and it opens up vast worlds (wow i sound like a book publisher now, sheesh) for you to explore.
Absolutely this! Reading is absolutely important. Someone else said read magazine articles and newspapers because the writers and editors would use perfect English. Yes read the newspapers and the magazines but they do not use perfect English. Read them to see if you can spot the mistakes. writers reporters copy editors and so forth are human just like you are however they have a system of squashing grammatical errors whereby the reporter does a write up then proof reads his copy. This is then sent to the copy editor for conditional proofing, and the copy may be read two or more times to make sure that grammatical structure is correct and to shorten if possible the work to fit into the space alotted. The copy editor then sends the proofed work to the editor who does a final once over and either rejects or approves the copy or requests changes be made and then the process is repeated until the editor approves it, then it goes to print and it STILL will have errors in it. Read books by everyone. Read it for the English but also read it for critical thinking. Expand your social conciousness with Elmer Gantry, or read Robert Heinlein to find out about a rocket gantry. Read Kurt Vonnegut. Read Anne Rand. Read Stephen King. Read as many authors as you can. Then write. Write everything you experience then read what you have written and see if it holds to the rules. If it does job well done. If it does not, still job well done as not every author follows the rules.
Have fun and enjoy!
Boring as it may sound one of the most useful grammatical tools for sentence structure in English (and in many other languages as well) is the plain old Sentence Diagram. Subject/Predicate Noun/adjective/verb/adverb punctuation voice and so on.
Word of the Day calendars, and websites are good to get a grasp on multisyllabic words and then Wikipedia and Wictionary are your friends for finding out how to pronounce a word and what the word means in context of other words or as a way to gain general knowledge about your world through differentiated subject matter.