What do you feel your programming level is?
I would suggest going with Keras if you don’t feel too confident. It’s quite limited, but I think you will find that you will be able to solve many machine learning problems.
If you are confident, then start using Tensoflow or Caffe. I would say tensor flow has more documentation out there, but I don’t have hard numbers on that. Caffe is what you should go with if you are going to be working with CNN’s. It is fast for image processing.