How can I stack a layer cake if the layers are round at the top?

I bought a box of cake mix and baked it in two 9-inch pans. I just took them out of the oven, and they're all puffy and weird-shaped. I tried pushing the top down so it'd flatten out, but it poofed back up! So like am I supposed to glue the two round sides together with frosting? Or like one round side to a flat side? Or should I chop off the roundish part so it's flat? I've never baked cake before, mostly just brownies and cookies, so I'm new at this. I'm baking myself a birthday cake, cuz baking's fun. I turned 13 today. Woo-hoo! Okay, can someone help me?