You can run PHP sites under IIS or Apache
If you already have IIS installed then you can run PHP with it - just google running PHP under IIS for more information.
If you want to go down the route of running it under Apache then here is a good straightforward guide to installing everything - it takes you through the whole set up process.
http://www.bicubica.com/apache-php-mysql/index.php
I would recommend trying to match your development environment as closely as possible to what the live one is going to be/or already is if the site is already live - it will save lots of hassle down the line.
I would recommend Eclipse as a free IDE. It doesn't come with a debugger however so you might want to look at Zend or XDebug.
If you are only going to be making HTML/CSS changes, you might not want to bother with Eclipse, just use your favorite editor.
W3Schools PHP section is great to help you get started getting to grips with PHP