Indesign and Data Merge

I have a card design double sided, it is for a child sponsorship in Haiti, I need to merge with a spreadsheet that has over 300 kids, I go through the steps and it either puts the same photo and info on every page or just one.

So my question is when I set up the InDesign Document how do I set it up, do I set over 300 Pages? Do I set up the copy on each page or will it place it on every page, do I leave all but the first page blank?

Can someone list the steps in simple language? Every thing I look up starts after the indesign document is created and I am I will add a copy of the layout.

It is a two sided document printed front and back 300 times plus.