There's a pretty good answer here that you may want to consider. It specifically is talking about at home exercises, so I'll let that speak for itself. If you need any help with what those exercises look like, check out exrx.net.
Is it possible to get some improvement using dumbbells only at home ?
Yes, although I think you'll find that a few other cheap pieces of gear will make a huge difference. Tack on a pullup bar and a kettlebell and you'll have a formidable home training system.
It is a mystery to me how people build their muscles , is there any
technique to follow ? Or is not possible for Obese people ?
If you want the most effective methods for strength training and you're starting from a fairly novice prospective, you will get phenominal results from something like Starting Strength or Strong Lifts 5x5. Those are barbell programs which will require access to somewhat expensive gear. You'll either need to buy it yourself, go to a properly equipped gym (most are not), or have a friend who has gear like that.
A properly run home fitness program can be very effective, but it will never be on par with a barbell strength training program (like the fore mentioned Starting Strength or Strong Lifts 5x5.).