URL Rewriting using .htaccess and mod_rewrite php

[91local]

I need help to rewrite my URL for better promotion.

My link is

http://www.domain.com/bb/cat_sell.php?cid=1

where cid=1 is Agriculture

Now i want url like this

http://www.domain.com/bb/category/Agriculture.html

and this is the ad posted in that category :

http://www.domain.com/bb/view_offer.php?id=37

I want url like this

http://www.domain.com/bb/selloffer/37/offername.html



I have web hosting with mod_rewrite on.

I tried using .htaccess file and placed in "bb" directory but it
didn't change.

There is no such directory liek category or selloffer but i want it in
the url.

Can any one help me the write process and what should be wirtten
in .htaccess file.