Read. Read often. And make sure you are understanding what you are reading. It doesn't have to be War & Peace. You could read a well-known newspaper, like The New York Times or The Washington post.
Step2
Keep up with current events. When you go to a party and someone asks you what you thought of last night's game, be able to give an answer.
Step3
Talk. Part of being intelligent is being able to convey your thoughts and ideas to others. Practice talking to people. If you're shy, talk to other people who're shy. Break the ice. It'll help you down the road.
Step4
Join a discussion forum online. There are literally millions to choose from. Forums are a great stepping stone to being able to convey your thoughts on a topic. And you'll find others who share your interests.
Step5
Act intelligently. For goodness sake, use your common sense. Don't be the guy (or girl) at the party who gets wasted. Be the designated driver. Don't cross the street when there's oncoming traffic. Use your brain.
Tips & Warnings
These steps are guaranteed to work, if you try hard enough.
Novice 
