Why cant i watch any videos on my mac?

What kind of videos are you trying to watch?

What OSX version are you using. You posted in the Lion forum, but if you just got a MacBook it would probably be running Mountain Lion.

Have you installed Adobe Flash. It doesn't come with Lion or Mountain Lion?

http://get.adobe.com/flashplayer/

What is the extension for the video that you are trying to play, i.e., .mov, .avi, etc.?

Also try: http://www.videolan.org/vlc/