When someone isn't up to date with current technology, fashion, or news, you can tell them to "Get with it!" This means that they should catch up with what's happening now.

Of course, this phrase is extremely useful when talking to your parents, because parents are always out of fashion:

Mom, no one buys CDs any more. You just download the singles. Come on, get with it!

Dad, cargo pants went out of style five years ago. Get with it!

The stress in this phrase is on "with":

Get with it!