If a fresher starts a marketing agency with no clients yet, what should they show their first client to build trust?

I am 16 year old Sarthak buliding my social media marketing agency I am very confusing what to show to a client so he hire me Becouse I am freshers i don't have any client yet So what i can show in my portfolio so can a client hire me

Take a example If I want to work with gym owners so what I will show in my portfolio to gym owner so he hire me