Four and a half months ago, I was 400 lbs and could barely walk around the block. 

My continuous weight gain presented me with a catch-22 because while I knew I needed to exercise, I was too tired to exercise and hurt too badly to perform the exercise I needed.
Now, some of my fatigue is due to my having four boys ages 5 & under, working full-time and completing my accounting degree at night.
However, I cannot deny that most of my problems probably stem from obesity. 
In moment of clarity, I had an "enough is enough!" moment. While I couldn't find time for the gym (and quite honestly didn't have the energy to go), and while my bruised feet, fatigue, and cardiovascularly challenged heart limited my abilities, I decided to just change out one habit for another: I would skip coffee on morning break and walk around the block instead.
Really, I wasn't burning very many calories at all. However, in that moment I was telling my body, "You are not my master. I control you!" In my life I've found that my lack of discipline in one area of life often bleeds over into other areas of my life. The reverse is also true. When I intentionally disciplined myself in skipping that coffee, it began to help me take control again.
It was embarrassing at first. I didn't want anyone to know I was walking on my breaks at work--although I'm pretty sure my panting and pouring sweat may have given me away--but I persisted anyway. I'd go for my walk, come back to my desk and try to cool down before I drenched my work shirt. Eventually though I worked up to 5,500 steps a day, then 8,500, then 12,000, and now I'm at 14,000 steps/day.
Fifty pounds lighter now, I can tell I'm getting stronger & faster. [Sadly, the sweating has not improved. lol]

This is not to brag, but rather to say, if you are discouraged or overwhelmed, just start small.  If you don't know where to start, just pick something and take control over that one thing.