Welcome to the forum Rochelle. Use this board and it will greatly aid you on your journey.
Taking it slowly is the sensible way to go. Permanent weight loss is about developing a healthy lifestyle that works for you in the real world. Getting that lifestyle takes practice and learning new healthy habits needs time to be established. So you are starting on the right foot...well done.
With the weight loss you will find that many of your health problems will disappear or became easier to manage as you progress. So hang in there and get ready for a better future. Sounds like your dog needs to develop a healthier lifestyle too
...time to move that mutt's butt!
Exercise will get easier as your fitness increases. Start very slowly at first. My first guide rule of weight loss is: do no harm. If you hurt yourself you do yourself no favours. Try a short walk at first. Swimming for you might also be the way to go. Just walking laps in the water could help. The water will support your weight and minimalise the stress on your joints.
Wishing you all the best