[Help] Registration modification and carrier update

[Artizzz]

Hi!

I'm planing to make delivery cost easier to manage(because in different cities have different delivery cost from delivery company what I'm using for delivery, that why I need to make easier for me that I write cost once and make prestashop add cost to invoice depending to place where customer take buy-ed products or live) and that's why I'm need to make when the customer in registration choose city from list like in country block, how can I make it? AND if customer don't live in city, he can choose in this list "Other" and then he he can write his place where he lives(for me it's OK that he writes place whats not city, because if he lives out of city cost for delivery is the same)...

P.S I need to know all files what I need to modified to get full effect.

And maybe anybody can tell how after that change, how can I make the carrier cost depends where customer lives? (I'm planing to some how add it to BO-> Shipping and make somethings like zones, counties, states, but actual problem from now is how can I make customer choose from all cities, but for help how can I make this to I will be very happy)

I hope I write clearly, because it's difficult to say everything in language what you don't know perfect that's why if something is not clear ask me.
And really hope can anybody can help me, because it's important for me.

Best regards