When are my potatoes ready to harvest?

This is my first year doing a veggie garden. I am confused by what I have read on growing potatoes. I planted them about two months ago and have plants about 12" high now. I keep reading about mounding and remounding them and I don't understand it. I just planted them in the ground like anything else. THat's what the book that I had told me to do. So what is the mounding and do I have to do it, I am so confused about this. I don't want to ruin my potatoes so I will do what I need to do. So my question is what is mounding, and do I have to do it and how do I do it? This is an easy 10 points. Thank you!