Redirect www.example.com/blog to blog.example.com

A lot of web hosts today provide an easy implemention for subdomain creation in their administration panels. You just need to to go there, choose you subdomain name, and then point it to a directory in your tree.
If you can’t, then it will be a little more complicated (You will need to resolve that subdomain to your server ip, configure some virtual hosts … etc) and you may not have enough privileges to do that (unless you are on a dedicated server).
RewriteEngine on
RewriteCond %{REQUEST_URI} !.
RewriteRule ^/blog$ http://blog.example.com [R=302,L]
but,  this is not enough by itself, you also need to change DNS, so that blog.example.com is pointed at the right server, and this can be done by a cname similar to this:
blog.example.com  CNAME example.com TTL 1080/or whatever your hosting company ttl
(not exactly how it will look, but use your DNS webinterface to set this up).