You have 2 options. Both require an adapter.
1, If your monitor has a digital DVI connection, (DVI-D not DVI-A), you could buy the Mini Displayport to DVI connector from apple, and buy a suitable DVI-D cable to connect.
2, You could buy the Mini Displayport to HDMI connector, and buy a suitable HDMI cable to connect to your monitor. This option will only allow you to run to a max resolution of 1080p however. The DVI has a much higher res output.