If you are running on a mechanical HDD with 1GB of RAM, 32-bit feels snappier. If you have an SSD, use 64-bit.
: While standard support for 18.04 has ended, some organizations still maintain these systems through Ubuntu Pro (Extended Security Maintenance), which can stretch support for up to 10 or 15 years for certain packages. ubuntu server 32 bit
Assuming you have legacy hardware and understand the security risks (i.e., this machine is or behind a strict firewall), here is the canonical way to install it. If you are running on a mechanical HDD