I can't get 3G on my iPhone 4?

I had a Samsung galaxy s3 that I could get 3G basically anywhere I went. I bought an iPhone, but I can't seem to get 3G at all. I can only access internet outside of my house if I'm signed up to free Wi-Fi. I have made sure that the 3G is on, but it just doesn't work. Any way I can change this?
Merry Christmas and thanks for the help!

Who's your 3G service provider? You should contact them.