There are 2 approaches you can take to purchase a resale HDB (assuming you fulfill all the HDB criteria).
If you are savvy and is willing to explore, you may gather more information from the HDB website to understand the entire procedure of purchasing your resale without an agent. HDB provides very comprehensive information on how to go about doing it. If you are going for this option, you do not have to pay the agent's fee.
If you are intending to engage an agent, the typical market commission for Buyer's agent is 1% of the agreed purchase price.
Hope this helps.