The most common way is to just ignore what is being said and going on about your business. A more demonstrative way would be to shrug (raise) your shoulders, with the rest of your body motionless. Stronger - shrug the shoulders and give a questioning look that says "Is there any reason I should be interested in this." Stronger - Shake your head and wave your hand away from the body. About as strong - look the speaker in the eye with a completely dead expression and say "You're kidding me, right?" Strong and vulgar - "Who gives a crap, anyway?" (Don't use this one!) For 'crap', a person might substitute several things, all vulgar, awful and undesirable. Use your imagination.

Who cares! ^_^

Be aware that some people may consider these impolite So what? As if I care? Whatever....... Talk to the hand [ raise your hand in a stop sign gesture] I don't give a stuff!

Fdmaxey's answer is wonderful (as always!) but here are some for a slightly different situation. When someone makes a statement that you don't necessarily agree with, but you don't feel like fighting, you can say "I guess (so)," "I suppose (so)" or even "I don't know." This is a fairly polite, un-agressive way to signal that you're ready to move to another topic (though, be warned, technically you are agreeing to whatever the other person says). For example, imagine someone starts talking excitedly about politics, but you just don't have an opinion. "The prime minister is insane! Because of his policies, the country is falling apart!" If you don't feel strongly about it, or don't want to talk about it, you could say, "I guess so," and change the subject.

A good example is when I get calls trying to sell things or want me to upgrade to a different cable or internet plan. The person on the other line is always trying to talk fast so I can't interrupt and say, "No, I'm not interested." I love to respond to their questions with one word answers and a completely unenthusiastic tone of voice. They know I'm not interested, but they have to keep reading their script. In a face to face conversation, doing things like constantly looking at your cell phone, avoiding eye contact, nodding your head in response to the things said like, "Yeah, ok whatever" all give the impression of indifference.