But note that usually you would make a concentrated ampicillin stock solution and add some volume of that stock to the media after autoclaving but before pouring the plates. You should not autoclave antibiotics.
Make up a 1000x stock solution of the antibiotic and filter sterilize it with a syringe filter. For example: 0.5 g ampicillin (sodium salt form, not free base) in 10 mL H2O for a final working concentration of 50 ug/mL. Store -20 C, and *thaw completely* and mix well when using. Autoclave medium with agar with a stir bar. When it has cooled to a bit less than 65 C, add 1 uL per mL of medium and mix (very slowly so bubbles don't form) for a few minutes on a stir plate before pouring. The plates have a working concentration of 50 ug/mL.