Since you said that it may be a basic question, I am assuming you are not talking of using a macro for highlighting specific words in a document. You just want to know how to highlight different portions of text.

I do not know how the menu is arranged in Office XP, but the instruction should be available in the Online/ Offline Help of Word. Click on the ? symbol on the top left corner of the Word window. In the Window that will open up, in the Search field, type "highlight" and hit Enter. You will see detailed instructions about highlighting text.