My mom passed leaving my 10yo as beneficiary to an IRA, company holding funds wants guardianship letter to release funds. How do I obtain this

This is a common mistake to name a minor as a beneficiary of any account.  Unfortunately, you'll need to file a peitition with the local court to have a guardian appointed.  Sadly you won't be able to be the guardian. Law prohibits that.  Someone else will have to be named or at least co-guardian with you.  I can't tell you how to file the petition in this QA forum.  Your best bet is to retain counsel to get this done quickly and correctly.