I know we all hate reading manuals, but this is still the best source of information. A little time spent in research will be less time in development later. The documentation is a little dry, but it will give you the background in standards and hierarchy. If you write to the standards WordPress updates are a lot less likely to cause you unexpected issues or even break your website.
If you really hate reading the documentation then at least look for a good tutorial to give you insight into the pitfalls. Sitepoint do a good one:
Certain files will override others if they exist in your theme. Concentrate on learning the structure of the files and the inheritance as them affect posts, pages, archives and your home page. If you understand which files take prescience over others, you will be able to problem solve, when your pages or posts are not displaying in the way you would expect.
Yoast do a great info-graphic on this:
If you are a real newbie then a starter theme or framework is a good way to learn good practice. I’m a big fan of Underscore. With solid standards and folder structures,it allows you to build a complicated and customisable theme, that will make sense to you have in a years time. They even have a generator to create the bare bones of your first theme.
It will be tempting to just insert references to your scripts and styles in your header or footer files. Don’t do it. Enqueuing them properly in your function file will save you time and frustration. You are basically telling WordPress what you are loading and will reduce load times and conflicts.
While you can put in an absolute url’s, when referencing images or includes, don’t do it. Reference the template directory, using the template tags, so that your theme works whatever domain it is installed on.
<?php include( get_template_directory() . '/includes/myfile.php'); ?>
By turning on the error reporting in WordPress while you are developing your theme in a test environment, you will pick up any errors in your code. These errors might not stop your theme from working, but they could cause bugs and conflicts later on. It is best to get rid of any errors before you use the your theme on a live website.
You can do this by adding the following lines to your wp-config.php file
// define('WP_DEBUG', false);
Just remember to comment them out, or delete them, before moving your website to a live environment.
Before you put your new theme on a live website be sure you have tested with real content and a few Plugins. This is your chance to find out if your code will brak when introduced to outside forces that you have less control over.
Even if you are working alone version control is a life safer. While it might seem intimidating at first Git is amazingly easy to set up. As you log each change with comments you will create an easy to follow development history. The ability to roll back code to a previous version is a life saver and can save you hours or days of coding if your changes completely break your code.
Yes I know, but be honest, you were going to ignore that one.