If you build a website then you should always make sure that it is mobile friendly. That a website is mobile friendly means that it scales and alters the layout to look good on any device without having to zoom. This might include stacking different columns on the website and making the text bigger so it easy to read on any device.
Why should I make sure my website is mobile friendly
A website that is not mobile friendly will be hard to read on handheld units. You will have to zoom in to see the text and you will usually have to scroll left and right to read the text. This causes a very bad user experience and most will leave if they visit a non-mobile friendly website on their mobile device. Traffic from mobile devices accounts for more than 50% of all web traffic. You lose out on a lot of potential customers by not having a mobile friendly website.
Your website will not be visible in search engines when some one searches for something on a handheld unit unless your website is mobile friendly. This means that the website will be invisible to more than half of all web traffic.
How do I know if my website is mobile friendly
A very easy way to see if your website is mobile friendly is to simply visit it in your handheld device. Does your website look differently than it does on your computer. Has it scaled to make the page easy to read? If so then your website is mobile friendly.
There are however a better way of doing it. If you simply look at it in your phone than you risk missing problems that reduces the user experience on handheld units. A better option is to use the tool Google provides that tests if your website is mobile friendly, Mobile-friendly test. The website tells you if the website is mobile friendly or not and if there are any improvements you can make.
Is there anything else I should think about?
It is always important that web pages loads quickly. It is even more important when a user visits the website on a mobile device. Mobile devices often have slower internet connections then computers and it is also common that users have data caps that limits how much data they can download. You should therefore always try to make the mobile version of your website as small as possible and make sure that it loads as quickly. Load speed plays an even bigger role as a ranking factor on mobile searches than it does for regular searches. Always use Google PageSpeed Insights to see if there is anything you need to improve. You should strive to have a score as close to 100 as possible. Anything below 90 should be considered unacceptable.
Google have developed the AMP framework to help you built fast mobile version of your website. You do not have to use AMP to build a website that will rank well on mobile units but the AMP framework will tell you which goals you should strive for. One such goal is that the website should load in less than one second. You can read more about the AMP framework by clicking the link above.