The most important thing to know is that the amount of usable storage space a laptop actually has is much less than what is advertised. It happens for two reasons.
First, your operating system is taking up space. The latest version of macOS, (High Sierra), for example, takes up a little more than 8GB, while the latest version of Windows 10 takes up about 15GB.
Second, due to the user-facing specs deal in normal base-10 numbers instead of the less intuitive base-2 numbers computers actually use, hard drives are just always smaller than they are rated to be.
Still, it is recommended that an extra 20GB for an operating system will be enough.