All really good points above.
In a standard compost heap, definitely avoid any cooked food, dairy, meat or fish. It will attract rats.
Grass clippings are good, but don't put too much in at once. Basically, for a decent compost heap you need a mix of green (sappy growth type stuff, veg peelings) and brown (harder wood, cardboard etc) type of stuff. They need to be mixed together. If you put too many grass clippings in at once, it becomes a sludgy mess because the air can't get into it.
If you go to the 'Articles and advice' tab on the main website and put compost in the search box on the right, you'll get lots of suggested links.
Here's a list of some of the main articles, including one on How to make compost.