How can I improve my ability to express myself and my opinions?

I would like to express very well how I feel about things- my opinions. I would also like to be more assertive, which I believe would fall in a similar category as with the rest. From careful analysis, I’ve concluded that the lack of a rich vocabulary may contribute to my being afraid of being challenged (and therefore avoiding confrontations where I may need to further express myself).
I would also like to evolve my critic side- I’d like to criticize more the things around me, and do it as well as those critics that most artists fear.
There is that proverb that goes: ‘The pen is mightier than the sword’. I would like to evolve myself, as a hobby, to this point, where I can use words to effectively defend myself (assertive with a touch of poison).

What are your suggestions as to how I may evolve this? If you also believe the lack of vocabulary is a problem, how do you recommend I solve this? What kind of books do you recommend? What type of things should I google?

As a side note, I will use this information to evolve my English and Portuguese speaking ability.