Smart people are not always born that way. It is your choice each and every day to learn new things or not. Once you have made a conscious choice to increase your intelligence, you have taken the first important step. While it is true that everyone may have different levels of intelligence, I believe everyone can choose to make their minds sharper and smarter.
The first thing I would recommend is to read. Find books that are about things you enjoy and read them. You will learn new words and your mind will absorb more than you think. Find one new word each day and look it up in the dictionary. Really learn it's meaning and you will find your vocabulary increasing quickly.
Do new things that are different than your normal experiences. Go to places like museums and aquariums and really learn about the things you see. Go online and look up something, anything, and learn all about it.
Talk to people and learn about different ways of life. Learn from their experiences.
Teach. I believe teaching others also helps us to learn. When you teach a child about new things, they will often teach you a whole new way of thinking about the subject that you may never have considered otherwise.