Here is a tutorial on how to create your own Wordpress Theme
Its not easy to do, and is not really that important. You can easily just download a free theme or purchase a premium theme which is simple and mobile friendly.
I personally wanted to create my own theme so it is streamlined exactly to my specifications, and does not use any resources that I do not need to use. This speeds up loading time, and lowers the bounce rates.
The plugins that I use change depending on what type of landing page I want to create. I tend to use as little as possible. If I need a contact forum I use the Contact Forum 7 plugin. If I want an Live Chat feature I use Tawk.to etc.
There is obviously something wrong with your setup. Below is a link to a reply I made which you may find useful.