If a defendant cannot afford bail, then at the arraignment or any hearing while still incarcerated the defendant can request a bail reduction or release without bail. That must be supported with evidence that the defendant is unlikely to reoffend or to flee.
The bail bond agent will need you to sign some paperwork in order for you to secure their services. This usually means paying an amount to the agent, usually a percentage of the total bail amount, and signing off on any collateral for the bond.