I feel too tired to exercise.

That’s ok.  There will be days where you just feel too tired.  But even on those days, small amounts of gentle activity will help you feel less tired overall.  Even if this activity is just a walk around the room occasionally, or a short walk to the end of the road, it’s still activity and is still good for you.  When you are feeling well enough, try to build up to 5 to 10 minutes at a time, setting goals to help with motivation.
 
But remember, if you feel unwell, or sore, then slow down or stop. If you are fighting an infection, running a temperature, or experiencing severe fatigue, then you should rest.