If a bill is sent to a collection agency will it go on your credit report?

Yes it will go on your credit report. It will usually appear as a "charge-off" if it has been sent to a collection agency. ***
Typicallly the answer is yes. It really depends on what the subscriber (the own of the debt) chooses to report. They may or may not report the debt to the Credit Reporting Agencies. They usually do, but I've seen it go both ways. If the debt is already on your credit report, get the debt paid in full (try to avoid a "settlement"). Then request (in writing, return receipt requested) that the original creditor report the debt "paid as agreed". This can positively impact your credit score. If consumers would take the initiative to communicate (in writing) with their creditors when they first encounter financial difficulty, the demand for collection agencies and bill collectors would significantly diminish.