If you don't mind reinstalling then you can always try, it might be easier.
I like separating my system in partitions, but I understand it's kind of old school. I already did that under dos and even win for years, but it can be a hassle for managing for a typical desktop user.
Anyway, when in the install you are at the partition step... If you don't mind losing everyhting on your disk (sda), which seems to be the case, use gparted (click on the edit partitions if I remember right to open it). then remove all your partitions on your disk.
Once all your disk is marked as being unallocated, create a primary partition. 200 MiB is enough, this one will be for /boot. You can name it if you want (label). I usually choose ext2 for boot, it's a very small partition so you don't need journaling.
Now create an extended partition with the all the rest of the disk (no need to choose a size it should be automatic). Then in this extended partition, create a logical disk. I like to put swap there. The size depends on your physical memory. Usually 2*RAM, so for example if you have 1GB RAM your swap should be 2 GB. Choose swap-fs as filesystem for this partition.
Next create another logical disk for root ( / ), size can vary but at least 10 GB. I'm fine with 15 GB, but if you install a lot of additional applications, games or whatever, more might be needed. Up to you. Choose ext4 for your filesystem.
The rest of your disk can be allocated for you personal data (My documents in windows, /home in Linux basically). So create another logical disk with all the rest. ext4 as filesystem.
Once all this is done you can quit gparted to go back to the install. In there for each partition in your disk, right click and choose Edit. There you can mount (
no need to bother with it once the install is done) all of them.
So as a reminder : sda1 mount as /boot, sda5 mount as swap, sda6 mount as /, sda7 mount as /home. No need to format except for sda6 (/) or the install will issue a warning. All done go to the next step in the install.
When you are asked where to install the booloader choose sda. Everything should be ok now.
With this setup your data is separated from your system files, If something goes wrong and you need to reinstall or recover your system you won't lose everything. The boot is separated too, I find it more convenient in case of rescue too. You can search more about partitions and filesystem in linux on google or in the mint forums. Another interesting advantage of partitioning is to avoid fragmentation which slows down your system, ext3/ext4 on linux fragments much less than windows fat and ntfs but it does happen. That's why you can add separate partitions for files that are created/removed often (mainly /tmp and /var). Still it is optional and might be too much of a hassle for a typical user.