Change code in Open Source Software to commercialize?

[qhnz]

I don't really have a good understanding about legal matter of open source. I read many online documents but didn't really find specific answers for my own questions. The situation is that I am planning to open an online business which requires to use open source content management software and shopping cart software. However, there is something I want to change in the codes to maximize the potential. My questions are:

1. Am I allowed to change codes and run the website of my own?
2. If the codes are changed, I still need to make notify of the original license, right? However, does it need to be visible for the visitors? For example: "This website is supported by Jooma.."
3. If it is open source, and if someone ask my company to see and reuse my modified version of the codes, do I need to give it to them?

Thanks and looking forward for good responses!
My struggling is that why Amazon didn't release its open source codes because Amazon uses a lot of open source platforms. I wonder if I modify to add in my own libraries or modules, do I need to release these. Since the libraries/modules are my own codes and it has nothing to do with the original software but just work as the "add-ins"