Question about erasing

Alright, forgive me for asking, but I somehow have missed how to use an eraser properly. I'm using PS CS2, so I'm hoping someone can help me. Chances are, it's really easy and I just havent quite figured it out. My question is about erasers. I see lots of tutorials saying to erase parts that block parts of the icon, and I have NO IDEA how to do the erasing. I always end up erasing everything in the icon. I dont really know where to start looking in the archives, so please forgive me if this has already been answered. What settings should my eraser be on? How should I have my layers? Thank you in advance for your help!