Just press the Windows key and type 'defrag'. The inbuilt defrag utility will be listed. But it is very limited in functionality and 3rd party tools are recommended. I use AusLogics Disk Defrag. There is a free version available too.
Automatic defragmentation is enabled by default in windows 8. So you don't have to do it manually unless you face serious disk related performance issues. You can change the defragmentation schedules if you want to. Available under disk->properties->tools->defragment now->configure schedule.