What circumstantial evidence are required to convict someone

I go to a grocery store and then there�s a guy standing next to me at the shop. When I left the store he found his phone missing. Next day when I go to the store he�s blaming me for theft as I was the only one standing next to him at the store at that point of time. These are the only facts he has. Now, 1. What circumstantial evidence is required here to prove theft 2. What defences will I have here.