What dictionary you recommend to build up vocabulary?

That would be a great way to build vocabulary. You should read alot of books and any word you dont know the meaning of, look it up. Thats the best way to learn new words. :) Also you can try: freerice.com this website is a fun way to learn new vocabulary while doing good for the world at the same time. (Go there and youll see why) Also if you visit a Barnes & Noble bookstore you will find countless books with worksheets and other things to help improve vocabulary.