Evolution - How did different systems of organs come into being?

I’ve just started reading some brief introduction to evolution on the web. It’s easier to understand evolution of individual organs due to mutation and natural selection. But how did different organs evolved in such a well structured and well coordinated way that it seems to be ‘built’ by someone instead?

Take the digestive system as an example: food gets in through the mouth, get processed and get out through a lengthy intestine. As it involves different parts of the body, it's hard to figure out how some organs evolved into being to fit others, especially those organs that 'know their way' and go around long length in the body, a system which seems to be ‘built’ by someone with a design in mind instead. I read from the web there is something like ‘information about the environment’, etc, and don’t know whether it’s relevant and if yes, how. As I didn’t study biology, would you explain it in an accessible way?

Note: I don't mean to start a debate between evolution and creationism. I just want to hear about how evolution works.