Adjusting Mobile width for responsive

Hi there,

I am struggling to adjust the width for my website to only go into mobile/tablet view from 767px and less. I found that on older PC screen set at 768px width, the entire website shows with a mobile menu.

Also, when in mobile view on my phone, can I change the slider not not be full screen, but only half screen?

I know it is two questions, but both related to mobile/responsive settings.

Please help - I have tried but cannot get it right.

My website is http://prefsol.preferredsolutions.co.za/ (still not 100% complete but getting there)