Nope is not completely interchangeable with no. You can only use it as a specific negative reply. 

For example. If your friend asked you if you have apples in your kitchen you could reply "nope"

BUT YOU COULD NOT SAY 

There are nope apples in the kitchen.

*when can I...

it has the same meaning as no, but it's used as a stand alone word. In other words you don't put it in the middle of a sentence. You would generally only use it as a one word answer. 

for example: "are you planning on going there?"

"nope"

Many thanks for those fast answers , it helps me so much :)

Yes it's the same but should only be used in very casual conversations, with a friend etc.  If your boss asked if you had finished an important job, it wouldn't be a great idea to answer "nope".  It carries no negative connotations other than being very casual. Same goes for "yep" or "yup" for yes.