There are many ways to improve your English skill. For example:
1. Reading dictionary in your spare time. This will improve your vocabulary.
2. Reading novels in English. This will improve almost everything (slang, expression, phrases, etc.)
3. Watching movies with English subtitle or subtitle in your native language. This will improve speaking and listening skill, it will also improve your skill to use expression or phrase when needed.
4. Chatting with stranger via Skype, Facebook, Twitter, etc.
Hope it helps.