Evangelism is not a calling, it's a command.
Jesus commanded us to "go and make disciples of all nations (Matthew 28:19)." This can be uncomfortable for those of us with a dark past. We can feel judged or ridiculed for sharing the gospel, especially by those who've seen us at our worst.
In Exodus, even when God is telling Moses to gather the Israelites from Egypt so they can be freed from slavery, Moses says "I am not eloquent (Exodus 4:10)."
Moses wasn't prepared or even equipped to speak about God in front of people (especially thousands of angry slaves as his first public address) yet he's who God chose to lead the Exodus.
There's no need to hide all that "dirty laundry" (Read: pissed jeans).
In fact, your dirty laundry is exactly what brings you down to a human level in the eyes of the people you're speaking to. It's what people hear most clearly and they'll be able to better relate to your story.
Don't only focus on the pretty parts of your past. A beautiful testimony is not what every human has in common; sin is.
Don't be afraid of who you used to be. Be encouraged by who God is moving you to become. Talk to people about Him and how He's moving you away from that pile of pissed jeans.