Thanks for asking a question on www.thisisyourbible.com.
Satan is not a fallen angel. The passages used to support this false idea are Isaiah 14 and Ezekiel 28. If you will take the time to read those chapters you will see that Isaiah 14 is a prophecy, in poetic language, about the King of Babylon and that Ezekiel 28 is the same about the "Prince of Tyre".
Here is a link to some information about the Devil and Satan:
Do You Believe in a Devil?
I hope you have found this helpful.