Manor Lords

You will need to make it to the Settlement Level (Small village) in order to build a manor which will then get you a retinue but a max of 12 only. You need 24 retainers, so you will need to build Garrison Tower by clicking on your manor and opening the Castle Planner to then build it. This Garrison will increase the retinue size by 12, increasing overall to 24. You will then need to Edit the retinue found in the army with it selected, click the paintbrush icon, and recruit Men-At-Arms for 50 gold, reaching 24 retainers, and unlocking the achievement.

To access your Retinue, you'll have to have the manor built. This requires a second area upgraded to a small village level. Once you have reached this point, build your manor, and open the army menu (V)

Select the Customize Retinue option, and purchase Men At Arms on the left of the screen. This takes 50 gold from your treasury each time, and you'll need to hire enough to max the Retinue out at 24.