Firstly, try running pacman -Syy, then try to install sudo again.
Check that the repositories are uncommented in /etc/pacman.conf.
Or your mirrorlist might be outdated: Generate a current list of mirrors and copy it to /etc/pacman.d/mirrorlist
Quoting from this relevant forum thread:
You can:
- pick another mirror
- try using an http mirror, not an ftp one (pick http mirror from the mirrorlist). Alternatively you can manually download the databases
wget > ftp://mirror.csclub.uwaterloo.ca/archlinux/community/os/x86_64/community.db > wget > ftp://mirror.csclub.uwaterloo.ca/archlinux/extra/os/x86_64/extra.db move them to /var/lib/pacman/sync/ and run 'pacman -Syu' again.
If you find any *.part files in /var/lib/pacman/sync/ e.g. /var/lib/pacman/sync/core.db.part - remove them.
To prevent having problems like these it is critical to understand pacman. To learn more about using pacman, see the ArchWiki pacman article, and consult man pacman.