I don't want my posts in alphabetical order

All my posts are in alphabetical order, but I want them in reverse chronological order (newest first, oldest last). But everything I find says that WordPress automatically does reverse chronological order, but mine isn’t and I don’t know how to fix it. I am using the twenty ten template, the one that WordPress set up as a default. The only thing I did was change the background color and add my own header. So….what is wrong? Help?