How do I free up stiff handlebars?

[TCPrince]

Hi. I have a Hero Ranger DTB bicycle and it's been a little over an year since I got it. I use it frequently but just recently, I've started having this problem.

When I start riding, I start to feel as if I have to put more pressure to make the handle turn. Slowly as I pick up speed, this seems more evident. When I go quite fast, in say 5th gear (rear), the handle just freezes up. And when I turn it with an effort, it shoots to the direction of the turn, in a very jerky manner.

Earlier, if I used to ride without having my hands on the handlebars, the bicycle would go just fine, and it won't go off-balance. But whenever I try that now, the handle just freezes up straight and doesn't automatically turn in the direction of my weight. This is just an observation which I thought might be worth writing.

How do I free up the handle? How do I make it as free and normal like it was before?

My bicycle: http://www.herocycles.com/rangerdtb1.htm

If you need a few close-up pics of my bike's handle or anything, plz let me know, and I'll respond as soon as possible.