How can I build up my muscles without going gym.I searched about it lot in internet but nothing helps me.I am 17yrs old now my wrist are thin I want to build it so help me tell me the natural exercise.

A good one for wrists and forearms is a weight, string and dowell rod:

Tie one end of the string to the dowell rod and the other end to a weight.

Now using both hands roll the string up, then let it out, roll it up and let it out.

Can you get the picture?

PS... some call this a hassle. Here are some alternatives discussed:
http://www.google.com/search?num=100&hl=en&lr=&newwindow=1&safe=off&q=workout%22string+and+hanging+weight%22&btnG=Search

A good one for wrists and forearms is a weight, string and dowell rod:

Tie one end of the string to the dowell rod and the other end to a weight.

Now using both hands roll the string up, then let it out, roll it up and let it out.

Can you get the picture?

PS...some call this a hassle. Here are some alternatives discussed:
http://www.google.com/search?num=100&hl=en&lr=&newwindow=1&safe=off&q=workout%22string+and+hanging+weight%22&btnG=Search


Tried to leave a comment but couldn't (spread it around factor)

I agree, we had something like the dowel-weight thing in our HS gym and you could really get a burn going sometimes. Also simple wrist curls, where you place your forearm on a stable surface with the hand over the edge, hold a weight, and flex up and down. Do this both ways, with the upper part of your arm on the table and then the under side of your arm on the table. This strengthens you wrist and forearms.

Also, one of those gripping devices is helpful.