Just do it!
I would recommend you see your GP or practice nurse about what is safe for you to do whilst obese. But there will be a lot you can do, and if you take the PT seriously (well, usually paying a lot of money really motivates one to take PT seriously
) you won't be obese for too much longer.
I wasn't hugely overweight but my BMI had crept up to about 28/30 a few years ago and at almost 60, it was hard to lose it. But working with a PT once a week, and committing to being honest with myself about tracking calories I lost around 13 kilos without too much difficulty.
My PT said to me straight up" You can't out train a bad diet."
I used the app for my FitBit to track calories, exercise & steps. Just trying to walk 15,000 to 20,000 steps a day is an excellent start. Throw away your car keys and walk.
YOu'll get really strong if you get the right combination of metabolic conditioning, weightlifting & other strength work, introduction to safe stretching, and high intensity interval training.
I've seen my PT working with very overweight women and he gets them doing what I do, but not such heavy weights or fast speeds. It's all good!
Enjoy yourself - a PT is great for forcing you to do what you wouldn't force yourself to do. I find that it's an hour of desperate hell, when I don't think I can run another lap, or press up, or push another 100 kilos, but my PT motivates me, keeps me going, and at the end of the hour, once I've caught my breath, I feel fantastic.