A well designed site will serve both desktops and mobile devices well and be fully responsive. Always target for the broadest audience. If you are getting into internet marketing your visitors will be a good mix of mobile and desktop. They may find the site on a mobile device but they will normally read the site and the articles, if they are truly interested, on a desktop.
Well designed mobile sites tend to leave off many of the images and charts, for speed reasons, that would interest someone looking to learn internet marketing, and most users know that. As wireless speeds continue to increase this will become less and less of an issue.
The first thing to consider before you design the site is what you are trying to convey with your site. What are you selling, what information are you passing to your users, what type of content are you looking to display, etc. These are important aspects of your site design. I can tell you from designing hundreds of sites, you will not get it exactly the way you want it on the first attempt. I have never had that happen with a customer site. We can normally get it pretty close but there will always be areas that will need to be tweaked as it goes on line.
If you wait to get the perfect design and layout, you will be waiting forever. All browsers and rendering engines view sites a little differently and it is those minor differences that will cause you to go back over and over again to tweak the layout and content.
Responsiveness is very important, as is speed, color, balance, layout and graphics, all of those other than speed have been discussed. Speed is becoming a very important aspect of SEO, so make certain you optimize your graphics and keep your code to a minimum to get quick loading times.
Security is also very important especially if you are selling items on line. Make certain whatever platform you use has good security features. One of the many reasons I would not venture into WordPress or any of the other big out-of-the-box CMS platforms. Their popularity has caused them to become targets for hackers, especially if the site becomes popular.
One of the biggest mistakes that I see is web design and web development individuals and companies promoting their services with a WordPress or Joomla, or many other CMS systems, as their website. Customers are pretty smart and if you do not have the skills to develop your own company site how can you develop the customer site. It is amazing how many individuals actually try to pass off a CMS template as their own site. With the ability to view the source of any site on-line it is very quickly determined.
There are many things to designing a good site. You should not expect to know all of them out of the gate. It will take you time to learn what works best for your audience. As your audience grows and you get feedback, tweak your layout, update your content and allow the site to grow.
A well designed website is like a living organism, it will grow and mature and develop based on the input it receives. I have been developing and designing sites professionally and teaching web engineering for over 15 years. Even today I am learning new aspects of web development almost daily, new techniques, new design patterns, new customer requirements.
Where I get the most enjoyment is learning from my college students. Fresh eyes, looking at an old problem, can come up with some really unique solutions.
Never stop learning and growing, everyone has information to share.