The big names in retail go to great lengths to secure their sites or mobile apps. Just be sure to always double check that the name is spelled right in the URL (so “amazon.com” not “amaz0n.com”) – small differences like the number 0, instead of the letter o could be the only difference.
It’s also important that the app comes from an official corporate site or an app store and is clearly labelled, and that the logo and colours match the brand.
If you plan to stick with browsers, look for the “S” at the end of HTTP in the address bar of your browser. (It sometimes appears only after you’ve logged in or clicked to the shopping basket.) If the “S” is not there, play it safe shop somewhere else.