How often should I use vitamin c serum as a beginner ?

Yes, it is beginner friendly 🙂 Since it's your first time using vit C product, start slowly anyway - two times a week in the AM and follow up with a sunscreen. Also, since your skin is sensitive, do a patch test first: there's citrus nobilis peel oil which can be irritating.

I use it every night.. I alternate between the two..

Once at night every 3days if you have sensitive skin

I think this vitamin C is very gentle but as a beginner you can start with every other day and once your skin terrorates it then you can use it daily.

Yes, you can use it😍