How to get the whole picture on Instagram?

So whenever I try to post a picture on Instagram from my camera roll, it comes up with this little box that doesn't fit the whole picture. How can I get it so the whole picture is in there and I can post it? My friend had said something about another app thingy that you use. Btw, I use the app on iPhone. What would the app be called? Please!

Turn the picture sideways after you take it and when you can edit it

You can't. One of the things many people like about Instagram is that it only allows square format pictures… Which means that if you want to edit a photo you've already taken with your phone, you need to crop it into a square (aka zoom in). I love square format, but if you're not smitten, there are other photo editing apps that let you use the whole picture; just search the app store.