How to be intelligent:
Be insanely curious about everything. The more you know about something the more curious you become. Be curious about everything from ants to the nature of the universe. You get want you desire, the desire of knowledge is called curiosity. Curiosity killed the cat is almost always said by ignorant people.
Do not stop learning, you will never know everything.
Entertain all ideas in your mind without immediately accepting or rejecting it.
Learn from a variety of sources of information.
Hang around intelligent people.