"Get a life" means to pursue something fulfilling or worthwhile. It is usually used in a disparaging way---for example, if someone is complaining about a petty issue, you might say, "For ***'s sake, Paul, does it really matter if you think the neighbors' new pink geraniums don't look good against their red house? " In other words, if the person has something meaningful to think about, he will stop concerning himself with trivial matters.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.