If you are referring to an unspecified quantitiy of something (in English one would sometimes use "some" in these cases), you use the article:
Je voudrais du thé, s'il vous plaît. -- I would like (some) tea, please.
Il boit de l'eau. -- He is drinking (some) water.
If you refer to a quantity (nothing being also a quantity in this case), you just use de:
Je voudrais une tasse de thé. -- I would like a cup of tea.
Il ne boit pas d'eau. -- He doesn't drink water.