What should I learn if..

Felix if you need to do this from scratch, learn PHP.. If your client is ok with using a CMS, then learn Wordpress it will save you a lot of time by not having to build the backend plus you get TONS of plugins that will help you.

Check out Wordpress trainings, the best out there.

Yes learn PhP.

You have option of wordpress or drupal. Both very good cmses. Your going to need SSL if you plan to do ecommerce.

I would say PHP definitely unless you go with an eCommerce platform. In that case I would say see what they use for their themes. They may or may not use PHP. I worked with one that used Smarty PHP templates and another that uses RTML. Either way I would say figure out whats going to work best for your business first and then choose the tools that make the most sense.

Braintree has a list of eCommerce Partners: https://www.braintreepayments.com/partners

For example, Bigcommerce: https://www.bigcommerce.com/pricing/

You could sign up for a free trial and see whats involved with modifying templates. FYI - as noted previously you would also need to buy an SSL which is most likely not included in that price. https://support.bigcommerce.com/articles/Public/Installing-an-SSL

You can use a WordPress plugin as well if you find a plugin that works for you. For example:

https://wordpress.org/plugins/tags/braintree

https://wordpress.org/plugins/wordpress-simple-paypal-shopping-cart/

WordPress is free and most plugins are as well although some plugins cost money and you may still need SSL. For that you'll need to learn install WordPress on your server and modify/build templates. You will definitely need PHP as the other guys said. Treehouse has a WordPress course.

Fortunately it wont cost you anything to look at your options before you choose. You can even grab a WordPress stack to practice on your local machine. Here is one: https://bitnami.com/stack/wordpress

Good luck

Thanks for the info guys! =)