Make sure your old iPhone is off. When I went from iPhone to Android, my iPhone was still on so even though I was sending texts to people when they replied their texts went to my old iPhone. This seemed to only happen with people replying FROM an iPhone.
Also, I would try to use the stock Samsung texting app at least to start with to rule out any aftermarket app issues. If you get the same thing on the stock messaging app then that would give us more info to work with.
I remember when I first went to Android it seemed a little confusing on some of the how's and why's, but after about two weeks it all made so much more sense than iPhone and was so much easier. Hang in there, you will be glad you did!