Using .htaccess to redirect to SSI or PHP3 ??

[rlingen]

My website is on a shared server which runs Apache on a Linux box. I have
hesitated incorporating SSI into my web pages as explained in the passage
immediately below, which was extraced from a recent email correspondence.

My comment:
> SSI or PHP3 will be the next addition to my website. Frankly, the only
> reason it's not on there now is that all the pages have to be named *.shtml
> or *.php3 for the server to recognize it, and I have had too many other
> things to do to change all my links.

My correspondent's response was:
As I understand it, you can deal with this VERY EASILY (on Apache,
anyway) if your ISP will allow you to run an .htaccess file for your
site. In that file you can specify all this and thus not have to
change any file extents.

An enquiry to my ISP (a duplicate of this message) was not terribly informative:
There is an .htaccess file in your

http://www folder

if you check there and add any
settings you necessary.

My question is: Is this possible, and WHAT settings need to be set? And
where can I read about them, in something close to plain English? And I
checked, there is no current .htaccess file in that directory, although I do have
one in another directory that is password protected.

I have full Telnet access to my site.

Thanks for any help.

Roy