Calcium - if you're a teenager, you need around 1000mg of Calcium per day. When you get a bit older, 800mg is probably enough and once you pass menopause, you should have 1000mg per day again. You should have a bit more if you're pregnant (1100) and more still if breast feeding (1300mg).
Generally, aim for 1000mg of calcium per day and you'll be right.
An idea of servings:
1 cup skim milk = 310mg
1 cup soy milk = 34mg (soy drinkers usually need supplements!)
1 cup fortified soy milk = 300mg
20g hard cheese = 150mg
20g cottage cheese = 14mg
200g yoghurt = 330-360mg (depending on the yoghurt)
50g salmon = 115mg
60ml custard = 70mg
2 scoops of icecream = 80mg (damn, wish it was more!)
Caffeine actually inhibits your metabolism of calcium so you need slightly more if you have your calcium servings with coffee.
Basically, three serves of calcium-rich products per day - like some milk, cheese and yoghurt should do it. You might need a calcium supplement if you don't think you can get 1000mg in a day. You can buy these from the chemist/natural food store, just make sure you get a tablet that has 1000mg of elemental Calcium in it. You should take it at night, on an empty stomach (so just before you go to bed) and don't take it near any caffeine products.
Hope that helped