Most important things to do with the website in 2018?

Hey guys, I am starting a new website that will sell Amazon products.

What do you think are the most important things and on what should I pay my attention the most?

Also, what theme do you suggest for something like this? I used Thrive Themes but I don't know now.

I will make a new thread with my journey in a couple of days so you can see me hitting my goal. ;)