Marketplace Website

Hi everyone.
i am new in UI/UX design world, but i always love UI/UX since high school. So far, i only doing it in game design, but right now i am doing UI/UX for Mobile Apps too. Now, i have project to design marketplace website for one company and i don’t know how to start. My client want website like, and i have couple ideas in my head, but i can’t pull it out when i start design it.
My questions are :

  1. What things should i consider before designing a website ?
  2. What theory should i learn to start doing website layout design ?

Thank you