You need to modify your torrc and use ExitNodes.
It allows the use of fingerprints (if you want to use a specific exit relay) or country codes, which is what you are looking for.
Note the following warning from the man page:
Note that if you list too few nodes here, or if you exclude too many exit nodes with ExcludeExitNodes, you can degrade functionality. For example, if none of the exits you list allows traffic on port 80 or 443, you won’t be able to browse the web.