how do I set up my parent's trust with an EIN number?

Most commonly, parents establish Revocable Living Trusts, in their names, using their own Social Security Numbers. Subject to the terms of the original trusts, continuing irrevocal trusts will likely require EIN numbers, separate accountings, reporting to beneficiaries, their agents, etc., as part of ongoing trust administration. To specifically answer your question, the original trust documents, and actions taken since it was established should be reviewed to determine the history and current status of the estate, With that, you'll be in a much better position to coordiante with tax, accounting, and other professionals to move forward with your process.