using iMac 2008 (display port) as external monitor with MacBook Pro 2013 (thunderbolt)
With both the iMac and Macbook off connect the computers with a Firewire cable.
Turn on the MacBook while holding the "T" key. This puts the MacBook into Target Mode and a firewire icon appears on the screen.
From now on you only need touch the iMac Keyboard and mouse.
Turn on iMac while holding the Option key. This allows you to choose the Start Up volume, in this case the MacBook. Use the left and right arrows to select the MacBook icon.
Press Return and iMac will start up with the MacBook System, Users, and Applications.
You can now close the MacBook and enjoy the "big screen".


To turn off.
1.Shut down iMac using the Shut down command in the apple menu (ie the normal way).
2.When the iMac is shut down. Press the power key of the MacBook to turn it off.

Hope this helps

There is no way of connecting the iMac through display port or thunderbolt since it is incompatible as stated in the comments below