What is the cheapest and safest way to travel between London & Paris?

I’ve never travelled to Europe. I’m thinking about a trip to either Paris or London. I’d like to either stay in Paris and go to London for 1–2 days or stay in London and go to Paris for 1–2 days.

So, which combo would you recommend…and what is a fast, cheap, and safe way to get back & forth?

I speak english & almost no french.