The best plan is to have what we call a root partition, for Linux and the programs, and a home partition for your own files. That way you can replace your Linux without affecting your data. Most people add a swap partition. That can be used by the computer if it runs out of memory (not very likely these days) or for hibernating.
Root partition: 10GB; mount point (the part of the filing system that lives there) "/"
Home partition: as much as you can get; mount point "/home"
Swap partition: as big as your RAM, if you want to hibernate.
If you're going to double boot with Windows, defrag the Windows partition first. The live disk of Mint will include Gparted, a very easy tool to create partitions. There's a screen-shot at