Drupal Yslow Performance Hello  I am using drupal 7  and yslow tool in ff.

it shows me grade B

Grade F on Make fewer HTTP requests

This page has 11 external Javascript scripts. Try combining them into one.
This page has 5 external stylesheets. Try combining them into one.

As i am not able to combine all javascript and  css files. 

I also enabled cache . but i don't know how it work. 

Grade F on Add Expires headers

There are 75 static components without a far-future expiration date.

i also made changes in .htacess file still not working .. Please let me know 

Grade C on Put JavaScript at bottom

i am using iframe in my website . How to include javascript at bottom for this?

Anybody have idea please let me know. 

Thanks
for Grade F on Make fewer HTTP requests

Locate them at admin &gt; configuration &gt; development &gt; performance. There you'll see two options:

Aggregate and compress css files.
Aggregate Javascript files