As soon as you can after you become a believer!
“Those who believed were baptized that day.”
“Then Philip began with the Scripture and told him the good news about Jesus. As they travelled along the road, they came to some water, and the man said, ‘Look, here is water. Why shouldn’t I be baptized right now?’ Philip said, ‘If you believe with all your heart, you may.’ The man answered, ‘I believe that Jesus Christ is the Son of God.’ So they went down into the water and Philip baptized him.”
The Bible teaches that there is no reason to delay your baptism. After becoming a believer, you should participate in the next available opportunity your church provides for baptism. If you wait until you feel you are a “good enough” believer, you will never feel ready for baptism.