To settle for: To accept something that is less that actually you would like to have. I wanted to win the prize, but I had to settle for the second position. (Conformarse con)

To settle down: Start living a quiet and calm life, especially when you get married. (Sentar la cabeza) My friends say that I will never settle down. To settle down means to calm down or chill out as well. When you use settle down TO... means to start doing something giving it all your attention. (aplicarse) I settled down to work after the meeting