Asalaam-o-alaiakum wa Rahmatullahi wa Barakatuh brother!
Masha'Allah... what an amazing gift Allah has given you! Never forget it. Welcome to Islam brother!
And yes, you better believe it, a non-Muslim saying shahadah with conviction = 100% clean of sins at that moment. Anything you EVER did before you were Muslim has now been wiped clean. That is Allah's promise and part of your faith as a Muslim now is to have 100% firm yaqeen (belief, conviction) in it. You're like a newborn baby. Enjoy it and cherish it! New life.
May Allah keep you steadfast in your imaan and may He make it easy for you to learn and implement this beautiful deen into your life, may He keep on guidance and bless you, your family, and your future generations until the day of Qiyamah. Ameen!
Please ask any questions you may have about starting to implement Islam into your life and the brothers and sisters around here will definitely help you out, Insha'Allah.