I'd be stating the obvious but you already know 145 at 5'6 is properly heavy. Every activity right now, no matter what it is needs to be gentle and, initially at least, limited. You started the diet two days ago so lets get you out, as you said, walking to begin with. Enough to get a sweat on and oh so slightly out of breath - at least 20 minutes. if where you live is hot take a damp cool towel and a little cold water with you. You're looking to slowly increase the speed and time. I wanna see 40-minutes in a few weeks. Swap, as you improve, from walking on the flat to walking hills.
Once you hit that target and weight gets dropped there are a number of machines you can use at the gym. Exercise bikes and so-called cross trainer type machines (the ones I'm thinking of you stand on and swing your legs back and forth while pumping your arms. Later on try a rowing machine. As with the advice for walking start easy and slowly increase the effort.
I'd expect numbers, rate and effort to take 8 weeks based on 2 times a week (increase this to 3 when you can).