To sign off is to end a communication. With business emails, there are some very basic and common ways to end the message. The only question is how formal or informal you want to be.
The last sentence of an email often says something like this: please contact me if you have any questions. This is neutral and you can always use it. To make it less formal: please let me know if you have any questions. More formal: please don't hesitate to contact me if you have any questions or concerns.
Closing the message, you can put: regards or best regards. More formally, you could put sincerely. Less formally: best or all best. Sometimes people from the UK will put cheers.
Want to practice? Send me an email titled "practice email" at email@example.com