1. This is in Apache's configuration (/etc/apache.conf iirc, or use a front-end configuration utility). The default should be /var/www/html
2. This one's kinda complicated. You could have a web-based front-end that would setup a user account on your system, but it would require CGI or PHP to work. "or do i need to do that" -- It depends. If you're setting up your server to be used by the general public, then yeah, but if it's just for your friends, and they can call you on the phone, it'd be much simpler to run "adduser <username>", then setup some web-based solution.
3. Honestly, no clue. You'd have to check the server's documentation. For security reasons though, I'd recommend SFTP, rather than FTP. SFTP works over SSH, and therefore encrypts it's inbound/outbound packets (i.e password data). To setup multiple users to access the same directory with it (and this probably would work with VsFtpD too), you'd simply setup each user with the same home dir (i.e "useradd firstuser -d /home/shared && useradd seconduser -d /home/shared"). I know there's a way to setup a chroot jail so that the user can't move outside their home dir as well (if you have perms setup correctly they can't write outside their home dir anyways, and shouldn't be able to enter other home dirs), but have never set it up myself.
Hope you get it all setup.