L.A.
http://www.google.com/search?hl=en&q=litter+trained+dog
There are also puppy pads you can just place on the floor, and train them to go on that.
Crate training a puppy really is the best way to go if you want to train it to go outside. And start it as soon as you get him/her.
http://www.google.com/search?hl=en&q=crate+training
There are lots of ways to do it. Just remember, that rat terriers are very active and will need to be taken outside daily to run around and get some fresh air anyways. Have fun, I've had dogs all my life and just love them!