Step 1 : Enable the trigger module if you haven't done so. (To enable the trigger module, go to administer->site building->module. check the trigger module and press save.)

Step 2 : Go the administration panel and click on action. Under Make a new advanced action available, select Redirect to URL... and hit create.

Enter a description and the url to redirect to. You can use internal url like node/123 to redirect the user on login.

Press Save.