I'm getting into making ecommerce apps. I need help and advice on Payment forms!!

I have just started freelancing and before accepting any payments, I'm going to make ten free websites for people.

Someone wants me to make a basic ecommerce site for them where payments take place online.
I need some advice on this.

1. How do you embed a checkout form in your site?
   I've looked all over the place and I couldn't really find a way. I personally don't have a credit card/bank account since I'm only fifteen.

2. How do I transfer the payment to the owner of the website?
   The setup of a Paypal account seems super complicated. What if someone is not familliar with technology? How will I link the checkout form to their bank account?

Any advice on the subject will be appreciated. I need help fast and urgently. Thanks!