Need Help Regarding Website

Hi there SEOs. Hope you are all good. I have a query regarding my website. Actually I am not much of a web developer. My budget is short so I got a few pages on my website built. Now I want a category page. The problem is I can easily create categories in WordPress but they come with their default design on the theme and I cannot edit that category page. I made another page but I cannot set it as a category page. So what am I doing wrong here? (I know am a newbie trying to work things out so your answers will be highly appreciated). Or is there any way I can fix this? What will be the impact on my SEO if I use a normal page yo list my articles instead of setting is as a category page? Thanks a lot everyone.