Be Sure Your Song Is Original

How to be sure that the song that you wrote is original...

Steps

Write your own lyrics. If you're using someone else's poetry, make sure that they know about it. If it's not someone you know personally, contact them before you publish or perform your song; you will need their consent. Maybe stray away from writing in more of the common themes in songs, like love, heartbreak, revenge, and materialism.
The music that you write will be heavily influenced by all the songs you've heard during your life. Play or sing your song to everyone you know, and they might recognize an existing song in your melody. As long as it's not intentional, you're alright, but your song will be better if it's more original!
Search a line or two on Google with double quotes ("") around them. This will search for the exact phrase.

Start out with a poem and add a tune to it!
Don't worry about other people's opinions too much. If you like it, keep it, but make sure it's something people can relate to.