First try to sync it, if that doesn't work look for a library section on itunes (should be on the left side of the screen),
click and drag the app/music e.t.c and drag it into the devices section where it says your itouch name.
If you have lots of apps/music to put onto your iTouch, hold Ctrl and click every music/app you want to put into your itouch and drag it to the device section
sounds a bit confusing though