for hardness on the feet/knees it goes:
treadmill
trail
pavement
sidewalk (hardest)
if you're just starting, try a walk/run program. start by doing a brisk 5 minute warmup walk, then do some light stretching. begin your run by running 1 minute, then walking 4. do this 6 times. the next week up the time to 1:30 run/ 3:30 walk if you're feeling up to it. as the weeks go on, increase your time as you see fit. don't get discouraged if you have to step back a week or remain at a certain time for a couple of weeks.
always stretch at the end of your run as well.