Will I ask for validation if I know the debt is mine?

Yes, you should ask for the debt validation. If they validate the debt, then you have to repay the debt. You can go for settlement. Try to consider lump sum settlement to save money.

Debt Validation is a time-Sensitive matter. Within five days of the first communication, the debt collector should send a written debt validation notice to you. Are you sure that the credit card account is yours? How much balance amount?
I would suggest you to ask for the debt validation asap.

Use this debt validation letter:
https://community.debtcc.com/letters/debt-validation.html

Yes, it's important to validate debts. Once the debt is validated, use this letter to settle your debts.
https://community.debtcc.com/letters/sample24.html

Will you settle debts yourself or through a settlement company?

That validation letter requests information that is not required by the FDCPA. The best letter simply states “I dispute the referenced account and demand validation.”