IPhone 4s Stuck On Landscape Mode When Texting?

Ok so when I text on my Iphone 4s it goes stight to landscape mode and I hate it! Please give a very simple way of how to solve this problem!

Press your button that takes you back to your home screen twice, swipe over to the left and there should be a little button that you can click on that will lock the screen so that your keyboard can't flip over. The button is like a lock with an arrow going around it