@ehuntington Figure out how to make a habit of going to the gym. Make exercise a priority equal to the most important things in your life.
Figure out a means to track your heart rate. For most people, this means buying a fitness tracker to wear on your wrist.
Go to the gym. Find every cardio machine. Pick 2-3. Treadmill for 7 min using the speed control to adjust your heart rate on the heart tracker to 100-120. Then switch to another machine until you have done 20 minutes.
How do you feel after? Nauseous, exhausted? Maybe back off the speed and time a little. But always have a goal that you want to beat, eventually.
The point is to feel accomplished at the gym, NOT hugely sore, nauseous, or exhausted afterward. If your brain associates the gym with exhaustion and nausea, it will convince you not to go. At some point, you will build up a series of goals and a feeling of accomplishment that will eclipse the potential of feeling wiped. Then, and only then, should you begin to go greater speeds and weights.
Edit: With some fitness trackers, you have to tell it you are exercising, or it will ignore or reach a ceiling. For example, your heart rate will be 120, and the tracker will still say 99. Most treadmills and cardio devices have metal contacts on handles that will show your heart rate. Use those to double-check your fitness tracker while you get to know it.