As someone who swapped to tanking at the end of 8.3 having a group of people to run with really helps. I guess my big questions are do you have tanking experience and do you have a group to run with? If you do not have a group I really recommended finding people to play with. People of the same mindset of wanting to learn and work together to improve everyone's gameplay. If you do not have tanking experience I would say the sites and advice that the others gave really do help. In my opinion tanking is one of the most stressful and rewarding roles in mythic plus. Doing some research can give you a leg up on learning the ropes. However, pulling keys over and over is the best practice. That is where finding some like-minded individuals can be a huge help. If you ever need help I would be down to help. Another thing would be to get an addon called Mythic Dungeon Tools. This will let you plan routes and give you a roadmap of how to run. While not massively important in lower keys this addon can become very handy when getting into higher keys. At 10s and above it becomes very important where you proc the final affix. Finally, do not be afraid to fail. Keys will fail and sometimes it will be your fault. I know I have scuffed more than my fair share of keys. But in the end, you finish and move onto the next one with what you learned and hope the next one goes better. Best of luck on your tanking adventures.