Help me regarding yoga...!!! Am a very beginner....?

Hi all... I want to start doing yoga regularly for fitness purpose.. I am 21 with 5.1 height and 50 kgs weight... tat seems to be a normal BMI... Bt my problem is I got belly and heavy thighs and ... i want to reduce it... my joints r so close tat it pains wen i do little exercise... so wat shall i do to open my joints and give me some asanas to reduce my tummy, thighs and ................. :-(