The short answer is you code first when the guidelines and code book instructions tell you to code first. In this particular case, if you look up I11.0, there an instructional note underneath the code that states "Use additional code to identify type of heart failure". Similarly, under I50.9 there's a 'code first' note that tell you that the hypertension code should come first.
In most cases (but not all), the convention is that the 'etiology' code comes first and the 'manifestation' second - in other words, the underlying disease is coded first, followed by the complications or diseases that were caused that first one. However, there are exceptions (for example in coding of anemia) and in some cases the reason for the encounter will determine the order. The guidelines at the front of the ICD-10 books are always the best first place to look when you have questions such as this, and if you're just starting out in coding I recommend investing the time to get as familiar as you can with the guidelines and return to them frequently as they will prove to be a very valuable resource for you in the future. Even after many years in coding, I still refer back to the guidelines almost every day to find answers to questions.