For desktop use, just go to Start, Settings, and Display. Look at the Multiple Displays section. Both monitors should be there, if not then click Detect to manually find them. You can now choose to duplicate the display (have two identical desktops on the two monitors), or extend (one giant desktop encompassing both monitors).
But that’s for regular, non-gaming, non-3D use. If you just set the dual screens up in Windows, they may work for gaming, as many modern titles detect such a setup and will go into windowed fullscreen mode. This will actually spare your GPU, as it will not be counted as two distinct displays. However, you will almost certainly not get 144Hz but rather 60Hz at most.
To ensure true, fullscreen gaming operation, the most important part of setup is via your graphics card’s control center. For NVIDIA, right click anywhere on your Windows desktop, go to NVIDIA Control Center, then Display, then Set Up Multiple Displays. You can assign a primary display, and detect screens just to make sure the right ones are listed.
With AMD, also right click anywhere on your Windows desktop. Go to AMD Radeon Software, then Display. You should be able to see both monitors listed. You can now create an Eyefinity profile, which may help alleviate some performance and compatibility issues. Go to AMD Eyefinity -> Quick Setup all the way down, and follow the instructions.