If you can boot from a USB key then you can create a start up disk from
Start-up Disk Creator (comes pre-installed in Ubuntu) or Unetbootin (you'll
need to install it from the repos). You will need a downloaded ISO for the
first option, but Unetbootin can download one and install from that.

If you can't boot a usb then you will need to burn the ISO to a CD or DVD.
The Ubuntu website help.ubuntu.com gives you help on both of these options,
plus lots else.

To start fresh you will want to format your partitions. Backup data before
you do anything unless you do not care about losing data.

Suggestion: Go with two partitions for Ubuntu, one for /home and one for /
(root). You will need to a manual or custom installation (last in the list
of choices). You need to know two things. Partitions need a file format
(choose ext4) and mount point (choose /home for home and / for root). Root
does not need to be large, 10 or 12 GBS is plenty for most people. Home
should be larger.

You only need to format /home once and you will preserve your data and
settings for as long as you like. It makes subsequent installations a
breeze. (In the future you choose manual installation again, and do
everything the same, except you do not format /home).