Not using debugging tools like Tag Assistant or Data Layer Inspector. Also, poor testing; Placing dataLayer = after the Google Tag Manager container snippet; Using too many auto-event listeners; Not using a proper naming convention; Tracking form submissions with click triggers.  

Google Tag Manager Community on Facebook; Google Tag Manager Community on Google+; Google Tag Manager Forums; Stack Overflow  

First, you need to have e-commerce data in the Data layer. Then you pass it with Google Analytics Tags to GA servers.  

It depends on what your goals are. Although there are a lot of new opportunities opened up for digital marketers once they start using GTM, sometimes developers' help is still needed.  

Yes, you can. You’ll just have to be more careful with setting up the triggers because sometimes you may only want to fire a particular tag only on website A but not on website B.  

There are two ways how to do that: By adding container codes directly into the website’s source code OR by using a plugin.    

If you’re a beginner, you have to learn all the Tags and their uses. The best ways to learn are through Google Analytics Academy Courses and watching YouTube videos.  

No. Google Tag Manager works well with multiple platforms/tools. It provides a bunch of pre-defined tag templates (like Google Analytics, HotJar, Twitter Universal Tag, etc), and you can add custom codes with the help of Custom HTML tags.    

Google Tag Manager has both Free and Premium plans. A free plan is more than enough for small and medium businesses. Large businesses can benefit from a paid Google Tag Manager 360 option.    

Anyone who wants to add/remove/edit various tracking codes on their (or their client’s) website. This usually includes digital marketers, web analysts, SEOs, PPC specialists, owners of e-commerce businesses, etc.