Go to Image in the menu bar and select Image Size. You can also hit Alt + Ctrl + I (Option + Command + I) on the keyboard.
In the box that pops up, be sure to check the box that says Resample. This ensures that Photoshop will create more pixels to add to the image.
There are a few different options for how Photoshop will perform this feat. Most of the time Automatic or Preserve Details work great. However, some of the others may work better with certain images so feel free to play around.