Here there are 3 scenarios:
1- You want to customize your current theme. You can do this by heading to Appearance > Customize in the left sidebar. You can customize your theme there, based on the theme’s design.
2- You want to change the theme’s structure. If you are familiar with HTML, CSS and PHP, you can start in WordPress Codex to learn how to use functions, filter, loops, etc. Then you can greatly customize your theme. Everything you need is in the codex.
3- You are new to web development, and you don’t have time to learn it, then you can either choose a free or premium theme, or hire somebody to design your theme the way you want. There are plenty of free themes in the WordPress’s theme store (Appearance > Themes) , or premium ones in places such as Theme Forest.