1. Open the Start Screen (Shortcut Key is Windows Key)
2. Type the file or app or setting which you want to search. Windows will automatically open the search page and type the text in search box which you provided.
3. Apps option is default. You can change it by selecting Settings or Files.
You should look at these articles:
Windows 8 Metro UI Tweaker
Windows 8 Snap Enabler
Pulmon: Windows 8 Start Screen for Windows XP/Vista/7
Windows 8 features for Windows 7 offered by Download Informer
Pulmon Start – Windows 8 Inspired Metro Based Start Menu
Zuro: Windows 8 Immersive UI and Metro UI Web Browser
Windows 8 Transformation Pack 1.0
Transform Windows XP into Windows 8
Get Windows 8 Metro UI Gadgets with Live Tiles in Windows 7 using Mosaic