What Vit. C & How to Use It (Beginners)

Does anyone know what vit c products can be used for a beginner with an oily skin tend to be sensitive? Beside the basic skincare, my routine already includes bha (paula) and retinol (inkeylist)🖤 I've always wanted to try out vit c but no idea to begin with, and when should to use it (day/night) I mean should vit c be used in a form or serum? Or like a toner; I've heard there's a products in a toner that has vit c in it. 💟 I just think beside my current routine, vitc can be helpful to brighten my face, like achieve a healthy bright, glow and youthful skin Any suggestion will be appreciated💕💕💕