First of all, would I be right if I said ASP/ASP .Net is the preffered/more popular choice of language for an e-commerce site?

The next thing, is I want to know how I can use PHP to make an e-commerce site.

I suppose the database (for the products) will be MySQL. I've been working on building database driven websites for a short while, but only for HTML content. Not e-commerce.

I have no idea where to start? Some tutorials/resources would be handy.

Thanks for any help/advice.

have a look here
http://www.phpwebcommerce.com/
and also at osCommerce and a quick search of php scripts sites will give you plenty to look at and get your ideas from

Right, thanks for the link.

The hardest part for me I think, will getting a system to generate the order and processing the payment. I'm not quite sure how I'm gonna do this.

My friend has asked me to make her such a site, I'd really love to do it, as i see it as a good oppotunity to improve my skills.

it's a huge undertaking so allow yourself a lot of time for learning, errors, and testing, testing, testing.