Pick what is going on. Get the steps in order, the official links, and the dates to remember.
Before a lender pulls your credit: temporarily lift (thaw) the freeze at all 3 bureaus. Mortgages pull all 3 (tri-merge). If it is still frozen, the pull fails and your pre-approval waits.
Official links
This page asks for no SSN, no account numbers and sends nothing anywhere. Only type your SSN on the bureaus' own sites or IdentityTheft.gov.
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Common myth: "Freezing my credit costs money, so I'll buy a monitoring plan." A credit freeze is free at all 3 bureaus (federal law since Sep 21, 2018) and stays on until you lift it.
Next steps
Pull all 3 reports free, every week, at AnnualCreditReport.com. Look for accounts and inquiries you don't know.
Found an account that isn't yours? Write the dispute letter in T234 (reason: identity theft) and enclose your FTC report.
Shopping lenders? Lift the freeze first, then keep the pulls inside the window in T237.
See your overall credit picture in T230, and what your score costs you in T37.