i wish to redirect

http://abc.com/a to /mypage.php?par1=a and 
http://abc.com/a/b to /mypage.php?par1=a&par2=b

how will it be achieved?

The next ought to be enough:

<IfModule mod_rewrite.c>
  RewriteBase /

  #so i doesn't match a real file/folder
  RewriteCond %{REQUEST_FILENAME} !-f
  RewriteCond %{REQUEST_FILENAME} !-d

  RewriteRule ^(\w+)$ mypage.php?par1=$1
  RewriteRule ^(\w+)/(\w+)$ mypage.php?par1=$1&par2=$2
</IfModule>