How do I approach revealing my past to new prospective boyfriends?

After 2 years of marriage and 2 kids I found out that my husband had married me while still married to his 1st wife. I have left him and nullified our marriage. I'm now in the process of meeting new people. When is a good time to explain these details? Online they ask about my kids etc. I feel like a have a huge stain on my reputation now and I don't want it to ruin my chances. What's a good way to explain this?