At the Start screen, just type your search word. A panel will appear on the right side of the screen, listing how many matches it found in different categories.
By default, most of the screen will display the results for the top category, Apps (which also includes old-fashioned Windows programs).
If you're looking for a file, and the word you typed doesn't happen to be the name of an installed program, the search will appear to come up empty. But check the Files listing in the right-hand panel, and hopefully it will display a number higher than 0. Click Files to see the files containing that word.
This looks so much like Windows 7, you'd think you could just click the Start button, type a word or phrase, and wait for a list of appropriate programs and files. But there's no Start button to push.
So you must use different techniques to search for a program or a file.
To find a program, press WINKEY-r, type the program name, and press ENTER. This doesn't always work. If it doesn't, you'll have to use the Modern interface technique described above.
You can look for files in File Explorer, which you can always start by clicking the folder icon on the taskbar. Once there, navigate to the folder or you want to search. File Explorer's default location, Libraries, will contain everything in your Documents, Photos, Video, and Music folders.
Click the Search field on the right, directly below the ribbon, and type the word or phrase you're searching for.
You can narrow your search with the various options in the ribbon's Search tab, which appears after you've done the initial search.