Can you walk without any discomfort or pain ? If you can walk, even for short distances, start with a daily walk. Walking is wonderful cardiovascular exercise and you don't have to do a lot to start. Also, do you do any type of stretching ? If you're stiff because you find yourself on your feet all day, stretching can help loosen up those joints. There are simple stretches you can start with posted in the Spark fitness section.
Starting slowly is the way to go. There are members who started by walking out to their mailbox and back. before long, they were walking around the block and eventually, they were walking miles. So, don't beat on yourself if you don't be able to do what other people seem to do with ease. You start with that first step and build from there.
I also recommend to think about adding some strength training to your exercise routine. You don't have to do a lot to start there either. All you need is a set of resistance bands so that you can do some band exercises at home. If you find standing difficult, you do upper body strength exercises while sitting in your chair.
Do what you can. With time and regular practice, your strength and endurance will increase.