Working Out + Cancer + Losing Weight + Where to Start?

Hello everyone, I just joined FitnessBlender and this is my first every discussion, hopefully of many to come!

I had been diagnosed with cancer in the thyroid gland many years ago and since had surgery to remove the whole thyroid gland which means that I am now on medication for the rest of my life because my body simply does not produce the thyroid hormone anymore. Because of this situation, I have since gained a lot of fat. Although I've not relapsed since, I want to put myself out of that worry and I want to start leading a healthier life for my children and myself. I am quite hesitant about working out due to the fact that I have no idea where to begin! I do not want to spend any money just yet, but I would like to figure out a starting point. My fitness levels are at the lowest they have ever been and I usually get tired very easily, so much so that when night falls, I feel exhausted even if my day hasn't been as active as it could have.

So my main questions are: Where should I start? At what intensity? What duration? And are there any key things I need to know before embarking on this fitness journey? Your advice is very much needed!