You need to install some sort of ftp server, try http://www.proftpd.org/ I think it's the best out there for your purpose, also there are lots of tutorials on how to set-up it properly and grant access for one user to certain directory. Let me know if you need any further help.
Depending on your distro here are a few examples of installation:
https://www.digitalocean.com/communi...n-ubuntu-12-04
https://www.digitalocean.com/communi...pd-on-centos-6
https://www.debian-administration.or...rver_on_Debian