These are known as Site Links and you have no control over them. If your site is new, you can't get any. On the other hand if your site is old enough or is a resource for some kind of valuable of quality information, Google will start displaying them. Pages which Google considers to be qualitative will be displayed. Moreover they are not permanent. Pages in the Site Links may change and also their number. Maximum number of site links that can be displayed at a time is 8