start with a jog around your neighborhood every morning.
diet is MASSIVELY important, so if you're actually trying to gain muscle you'll probably want to work on that too. you can't be eating stuff like chips and whatever all the time. soda is terrible too. drink water and milk.
do a few sets of pushups each day. start off with doing as many as you can, (hopefully at least like 10? 15? idk)
also if you're interested in abs (which personally i dont care about that much because literally all they do for you is make you look good) then just do situps each day and whatnot
then eventually you can begin doing a real workout plan, with each day consisting of a different thing you want to workout. arms, chest, legs, (yes, legs) all that good stuff.
but remember, diet is IMPORTANT. work on that first.