Welcome to the forum.
Adding materials is quite easy but as a new user, you would do well to go through all the tutorials available here:
http://www.graphisoft.com/education/training_guides/
While they are downloading, here's something from the help files to keep you busy:
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Go to Options > Element Attributes > Materials
1) From the pop-up palette at the top of the Material Settings dialog box, activate a material which looks like the one you want.
2) Click the Duplicate button.
3) Assign a name in the Duplicate Material dialog box.
4) Edit the material’s characteristics using the relevant controls in the panels of the Materials Settings dialog box. (The panels appearing here depend on the rendering engine you have chosen as the Preview engine, using the Create Preview with control.
5) Click OK to verify the new Material’s settings.
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There are finer controls you can experiment with as you become more familiar with the software and its settings. For the moment, though, I hope this helps.