How do you put songs on a Sansa c240 MP3 player?

If you plug the player into a USB port, it should appear in My Computer.
Open it and drag and drop the songs into it.
If you have a microSD card fitted that will show up as well.
Think of it as just another thumb drive.

You can also put songs on a microSD card using a card reader, and plug it into your player.

There are software utilities that will automatically update your player's contents, but i have never looked at them, having found the above two methods sufficient.