How do i make a resource only usable in a specific city's surrounding without the other cities being able to use it
Usable how?
For resource-requiring buildings, you simply check the box that says "Required resource must be within city radius". That way, any town which doesn't have the required resource with the 20 tiles it could work, will not be able to build that building.
For units, if you want e.g. Swordsmen to be only buildable near Iron, you can either (1) make Swordsmen non-buildable, but instead autoproduced by a building which requires Iron-within-the-city-radius, or (2) apply one of the more recent versions of the Flintlock patch (for the GOG/Steam edition of C3C) which limits where units can be built, based on local resource availability.
Either way can be exploited by the human, though — by founding up to 4 towns directly adjacent (C1 pattern) to the resource (R), or up to 6 towns within 2 tiles of it (C2 pattern), like this:
x . C2 . x . C2 . x
x . C1 . x . C1 . x
C2 . x . R . x . C2
x . C1 . x . C1 . x
x . C2 . x . C2 . x