you mentioned backup
the c/ drive has 21 gb used space
if i want to back up to a usb drive
how many gigs should it be?
what is a "backup?"
A backup is essentially making a copy of file(s) so that if the original is lost, destroyed, or damaged, you will still have the copy (aka "backup").
Typically, in the case of the OS and applications, you in theory should have a backup in the form of the original install discs (unless you downloaded the installer file to your hard drive...but even in that case with an OS, you typically need to make a bootable install disk or optical disc). So, at the bare minimum, it is usually advisable to make backup of any of your user files (i.e. files you create, edit, download, etc) that you care about. But, it can get much more involved.
If you want to make a disk image (essentially a backup of how your computer is setup at the time of the backup that would then allow you to restore the computer to that exact state) backup of your computer and you have only about 21 GB of used space, then you likely would want at a bare minimum a 32 GB flash drive (flash drives tend to jump from 16 GB capacity to 32 GB capacity). This would easily backup your current data and allow for around 11 GB of future data. If you think you might accumulate a lot of data (i.e. take a lot of pictures or record/download videos) and want to be able to back it all up, then you will likely want significantly more than a 32 GB drive. And realistically, I would tend to suggest a external USB hard drive rather than a flash drive...and while you can get external USB hard drives with less than 1 TB of capacity, the reality is that 250 GB or 160 GB external USB hard drives will cost almost as much (if not more) than a 1 TB external USB hard drive. In the US, you can get a 1 TB hard drive for about $50, but 160 GB or 250 GB hard drives are maybe $15 cheaper but many are actually more than $50. This dynamic might not be true in all other countries, but I only have experience with US pricing.