Great question, actually. What got me moving was my doctor saying that I need to go on maintenance medication to get my triglycerides down - at age 39. That drove me nuts and I decided to take a 3 month membership. Never looked back and gradually increased from 3 days/week to 5 days/week.
Whatever gets you motivated to start is great. I agree with Stl Bills - once you get over that initial hump, you will cruise. Tackle any potential mental excuses early on so they don't bother you in the long run. Remember, this is not a 1 week or 1 month deal - this is how you will want to lead the rest of your life.
I used to think I am too tired to workout in the evenings after dinner. That turned out to be a BS excuse. I usually go between 8-8:30 pm and sometimes finish when they close at 10 pm. No tiredness, no nothing. If you are a morning person (which I am most definitely not), then workout in the morning before going to work. If late evenings is not possible, go to gym first before going home. There is not set prescription or timing - I emphasize do what works for you.