For hard inquiries, call first since the process has changed. With TransUnion, you used to just call. Now they require you to report identity theft and send a letter with proof of that report.
For collections handled by a third party, send a debt validation letter, then ask for authorization for them to collect.
For collections with the original creditor, try to settle if you can’t pay, but you can ask for proof. Look for errors to dispute. Before paying or settling, get a written promise that they will remove the collection fom your credit report.