A bit more info would help, e.g. year, engine size and type, petrol or diesel.
This is usually caused by a faulty or dirty idle control valve and throttle body.Removal and cleaning depends on type.Cut and paste the following urls then use the sites search function for more info. You will have to register to use VVOC but it's free.
http://www.vauxhallownersnetwork.co.uk/index.php
http://www.vvoc.com/forum/forumdisplay.php?f=74
veccie
February 2009