PHP will increase load times... a static html file will always load quicker than one dynamically generated with php (or any web scripting language for that matter). There's thousands of reasons of why to use php, but load times is not one of them.
That said, i think you should start off by reading the php manual at php.net... that should give you a basic feel for what php can do, and what you can use it. Then, you'll probably need to evaluate what you want your web site to do, then see if it can be done using php.
imo, think along the lines of dynamic content, the ability to access data sources, where content can be served on a personal level with your user. Or even as a way to handle those "contact us" forms that are all over the internet. PHP isn't for design, in fact, it's encouraged to seperate the design from php (an article on that topic right here at phpbuilder).
Until you know what you want to do, i don't think you'll get much more help.... as Shadeless said, your description is kinda vague...
hope this helps
