Firstly check the mercury on the temp guage on the front of the aga, it should be on the black line in the centre.
Check it is here for 3 mornings in a row when you come down for breakfast. If too high or low, adjust it. And then DONT TOUCH THE TEMPERATURE.
The top oven is very very hot. But is less hot at the bottom of it. So roasties and yorkshire puddings, cook near the top. I put roast chicken and other things in their tray on the rack near the floor of that oven. Good place to put ready meals too.
If things are overheating and browning too quickly, that is when you need the cold shelf. You put it 2 rungs above, and it cools the temperature down, lets it carry on cooking without burning, make sure you keep the cold shelf away from the Aga, so that it is cold when it goes in!
If you have a 4 oven aga, the bottom oven works at about 3/4 the temperature of the top, ie mine is about 160 degrees. If a 2 oven aga it will be half. Which one do you have?
It is trial and error! Took me a while, but I love mine now.
A good book is by Sarah Whitaker called Relaxed Aga Cooking, or best of all try and go on an Aga class, Aga shops hold them or if you are near Berkshire, try www.treetops-newbury.co.uk.