Building a shopping cart?

I had a client ask me the other day if I could make a shopping cart on his site - he has 20 products he wishes to sell. I said I’d look into it so here I am. I am pretty clueless really. What advice might you give me?

Something else that bothered me was this - if someone from china wanted to purchase something - how would the prices be worked out? would we have to do it, or do shopping carts have a system that works it out?

Thank you.