I know how you feel Istanbul
I started cycling agian after many years in jauary...I was 5'9" and 255lbs. And extremely unathletic...a climb up a flight of stairs and I was getting light headed.
So far I'm down to a more tolerable 230..
just keep at it man, it may suck badly at first, but after time you will get stronger and that strength will help you get through the distance.
Carry LOTS of water at first. I still usually carry 2 16oz bottles of water just for commuting. When I'm out just to ride, I double that. Also consider getting slicks if you are riding a mtn bike or hybrid. It makes things a bit easier since you aren't fighting the knobs on your tires as well as allow for better stability on road.
As far as what I eat, my diet was restricted carbs/fat for the first two months, then restricted carbs for the thrid, now I just eat normally (jack in the box, pizza, tons of breads [i'm a carb-a-holic, it takes weapons to part me from my garlic bread!]). I was pushing about 5-8 miles a day for teh first two months, then 8-10 miles a day for month 3. Now it's at least 12 miles a day. Hopefully the way i chose might give you some insight in how to deal with it. Given I do take things a bit extreme so this might be too hard of a workout for some.