If you install Ubuntu as the only operating system on the hard drive (install option "use all of the hard drive" then it will repartition your hard drive and format it completely. It will warn you of this, however you have to read the screen carefully. You should back up all of your important data to an external hard drive from which you can recover your data after you have installed Ubuntu.
Ubuntu/Linux doesn't treat hard drives and partitions the way Windows does - there are no drive letters (C:, D:, E: etc), instead all storage devices (hard drives, USB pendrives etc) are given a letter scheme in a device name so that the device names become /dev/sda, /dev/sdb, /dev/sdc and so on. Furthermore, each partition on the hard drive is given a number so the first hard drive with say three partitions would be /dev/sda1, /dev/sda2, /dev/sda3 etc.
These drives are all mounted seamlessly within the / (root) file system, so the main boot drive is usually accessed as /, while a separate partition used to put all user data on will usually be mounted to /home.
You don't really need to know how to partition the hard drive. As soon as you tell the installer to use whole drive it will partition it and do all the set up for you.
If you select "do something else" at this point instead then it's possible to specify partitions, and even move partitions up or down, and resize them. It would be quite possible to copy all of your important data onto one of the other partitions and leave it there until after the installation, and copy it to your /home afterwards and re-use the space to grow your main data partition to take up all of the hard drive. You would need to move the data before you start the install; the partitioning program will not move data from one partition to another. This would be an advanced topic however, and I'd still strenuously recommend that you back up all of your important data to an external drive.