You had to hear if from someone, right?
You've heard things like this:
"It's the sun."
"The climate has changed before."
"Last winter had record cold spells."
"CO2 is not a pollutant."
or simply "It's not happening."
These and 119 other misconceptions are addressed at this site: Arguments
Just scroll down the list until you find objection to climate change that you've heard, click on the short answer in the right-hand column, and you'll find a complete explanation of the misconception.
And the site treats misconceptions from different sides of the argument. You'll also find three levels of detail in the answers: Basic, Intermediate, and Advanced.
Check the list no matter what you think about global warming. Talk about what you find below.