PHP MySql is probably best being free and plenty of online help for them.
As for an ecommerce solution you may be better with an off the shelf such as oscommerce.
Lastly for payment processors paypal is easy to use, also protx is a good one with a very thorough testing system in place before you go live.
I would recommend picking up one of the dummys guides or something similar although there are plenty of tutorials online if you have a quick search in google, stick in something like "php/mysql basics" or "php tutorial"
I'm more than happy to answer any q's so PM me if you wish
(I currently work as a web developer so I should be able to help.)