If you're ready for the agent phase, your 1st step is research. Go on QueryTracker and AgentQuery and find agents who represent your genre and are looking for the type of work you have. Check their track record to see if it fits within your plans. View each one's submission requirements, as everyone has different requirements. Write your query letter. If you're lucky enough to get feedback, listen to it and revise before continuing.

https://querytracker.net