When you "forget to do" something, it means that you wanted to do something, but you didn't do it because you forgot:
Oh, I forgot to lock the door. Hold on just a second.
Oh shoot. Remy called me yesterday and I forgot to call him back.
Notice that there is a different phrase for when you do something, but then later forget that you did it. That phrase is "I forgot that (clause)":
I forgot that she was a year younger than us. I thought we were all the same age.