Learning words is not just about learning the meaning of them; it also means you should learn well how that word is pronounced and where it can be used. Pronunciation is important in English so spend some time learning the right way.
Another important thing to remember is which words are formal or informal; spend some time learning about this as well; using slogan and informal words don't suit you well in professional environments e.g. client meetings perhaps.
Some good ways of improving your vocabulary:
- Read good novels
- Read articles related to your business and profession
- Listen to the words used in your environment e.g. train stations, workplace, etc; words others use frequently.
- Watch movies and pay attention to the words used
- Pay attention to the advertises on the street
One of the easiest ways is to learn words by their pictures which makes them easier to remember:
- E.g. use google images to find images of a particular word e.g. http://www.google.com/images?q=bathtub
And at the end, don't blame yourself if you don't know some words; there are many things others don't know either. Be patient, enjoy learning and practising gradually and consistently.