Things you can add to your compost heap:
Kitchen waste - vegetable and fruit waste, tea bags, coffee grounds, egg shells, dead flowers
Plant material - see below for execptions Wood ash
Soft prunings from the garden - chopped up if possible and not thick and woody or they will take years to compost.
Plant waste - things which don't come into the catagory below.
What not to add to your compost heap:
don't add these unless you want a garden full of them! It is very beautiful though.
Likewise the roots of pernicious (good word) weeds. The composting probably won't kill off the roots or seeds and you will be adding problems to your borders for later. So no bindweed, dandelion, brambles, celandine.
Cooked food. It does rot down but may attract rats.
Large twigs or woody growth. Too slow.
I don't use any chemicals so I wouldn't want to add grass clippings or any plant material which had been near weed or moss killer.
Diseased plants. Up to a point you need to know your disease as some will be not survive on dead plant material or through the winter (ie powdery mildew). Soil borne diseases will persist though. ( Onion white rot, clubroot etc). If in doubt, though, leave it out.