Ernest Hemingway used to write in a fairly simple, straightforward style. Love or hate him (people seem to feel one way or the other about his work), you can't deny his talent.
Vocabulary can be improved by reading. Everything you can get your hands on that's in English. Newspapers, books, magazines, cereal boxes. Anything. One thing you might start to observe is the differences in style. A lot of this has to do with the intended audience. We speak to children somewhat differently from how we speak to adults, and the same is true of our writing.
Teach yourself new words. Go to
www.dictionary.com and
www.thesaurus.com and look up words you don't understand at the first and then put them into the second site and you'll learn some synonyms for that word. And look up those words, too. There are very few truly pure synonyms in English. Rather, the words are close but have differences in shades of meaning. Here's an example.
Let's say the initial word is
cow. You know, the big farm animal. Well, English has these related words:
bull
calf
heifer
cattle
beef
ungulate
ruminant
Guernsey
They're related but not the same. Try looking up any of the unfamiliar words and you should be able to see what I'm talking about.