Learn Dutch: Kijk eens!

Kijk is a command, meaning "Look!". In Dutch, you tell someone to do something by using just the stem of the verb (the dictionary form minus -en). So, kijken becomes kijk. You use this exact same form whether you are talking to one person or a group.

Doesn't eens mean "once"? Why is it translated as "just" here?

You are right that eens literally means "once". However, the Dutch often use it as a friendly filler word to soften a command. Instead of a bossy "Look!", adding eens makes it sound more encouraging, like "Just look!" or "Take a look!".

Will I hear Dutch people pronounce eens exactly as it is spelled here?

Usually not! In everyday spoken Dutch, the word eens is almost always shortened when used this way. You will typically hear people say Kijk 's! where the second word sounds just like the English word "is" or a simple "s" sound.