Where and how many .htaccess files?

I’m wanting to change to pretty permalinks on an existing blog (actually a number of them). I get the “if your .htaccess file were writeable” message when trying to change. It appears that I must edit the .htaccess file myself since changing file permissions doesn’t work.

I am running a number of blogs under the same account and appear to have but one .htaccess file. Is this the one that I need to change? Or do I need to create and .htaccess file in each of the subfolders under which I have WordPress installed?

Also, do I replace the existing coding with that provided by WordPress? Or do I just amend what is there?

A note: since “pretty” Permalinks are better for SEO, wouldn’t it make sense to make them the default rather than the “P=xxx” format? I’m only figuring out now that I should have reset permalink BEFORE beginning my blogs, not now!! I’m not a programmer and am VERY nervous about changing machine files.

Thanks!

Kristin