5 Answers

Take an English course at a community college. 

In addition to taking an English course, listen to the news.  Most news anchors speak very good English.

Staying focused and looking up any word you don't understand or have some question of in regards to it's actual meaning. Not exactly an overnight process, but one that will strengthen your comprehension and understanding of many things. You could also take the words you do look up in a dictionary, and research them in a thesaurus for common usage, and similar meaning words. Make it a routine to do that with 3 to 5 words a day.

Yes like Shootah says the Theasaurus get a Roget's and a Collin's.

Spend a summer with Umbriel, perhaps?