How do I make a padlock on my browser appear?

Okay, so, sometimes when I go to some sites, the top of the browser (search sites, the one near the left and right symbols) will my highlighted green and a green padlock will appear near or on the arrow pointing down, so I want to buy something online but, I want to be extra safe that no one can hack my computer and get my credit card info, personal info, etc. By the way, if you need information, answer this question saying, "I need more info", or something like that. Then check back in maybe 30 minutes and the details will be added with more information as needed.