To keep this idea very simple...
Start small, keep it sustainable, and build over time.
9 times out of 10 I see people jump into a fitness routine that is wildly different from their existing lifestyle... this will only last so long.
The first step is different for everyone. It might be taking a walk outside, building a foundation with bodyweight exercises, training 3 times a week at home or the gym...
But whatever it may be, it has to be sustainable. Not necessarily comfortable, but sustainable.
A fitness journey should be lifelong, a part of your day-to-day lifestyle that you adjust and adapt to suit your needs at the stage of life that you're in.
Have a basic plan in place that covers the fundamentals, and perform it really well - as you evolve and improve you will learn to love the journey and find the movements more comfortable.
As this happens, build, build, and continue to build. As long as you continue to build and push yourself beyond the session before, progress will follow - and the sky is the limit.