See https://web.dev/responsive-web-design-basics/
Like janet4now points out, your parent theme probably already does all of this. Using the same styles as the parent is a good suggestion, but I’d go a step further and build my template off of the parent theme’s, using the same div/class structure so that the existing presumably responsive styles will be applied.
Of course at some point you need to deviate from the parent template, or there’d be no need for a child template ?? Use your parent theme as guidance in creating media queries to style according to screen width. Try to use the same breakpoints already used by the parent. Your browser’s element inspector developer tool is very useful in examining what the parent theme does for different devices.