Cycles is bundled as an addon that is enabled by default. To use Cycles, it must be set as the active render engine in the top header. Once that is done, interactive rendering can be started by setting a 3D view editor to draw mode Rendered. The render will keep updating as material and object modifications are done.
To see if and how you can use your GPU for rendering, see the documentation on GPU Rendering.
Here you can find some tutorials for Cycles.