Install and configure VSFTP given as per given at below link.

https://www.digitalocean.com/community/tutorials/how-to-set-up-vsftpd-on-ubuntu-12-04

As you want to give access to a particular directory, please find the below step after installation of VSFTP.

By changing User’s Home directory

Make sure following line exists /etc/vsftp/vsftpd.conf

chrootlocaluser=YES

Set User HOME Directory to /var/www/html, if you want to change for existing user then you can use :

usermod –home /var/www/ username

then set required permission on /var/www/html

Kindly follow the above step and do let me know in case of any query.

Dear @omarfilip
I think, you might visit this page: How To Install WordPress with LAMP on Ubuntu 16.04.

It can maybe useful for you.

Thank you!