To be honest, we don't really know what the shape of the universe is - that's because we can't see all of it! You may know that our universe hasn't existed forever; in fact, our universe is about fourteen billion years old (we don't really know what happened "before" fourteen billion years ago, or if it even makes sense to talk about what happened "before", but physicists have a few different ideas for what might have been). But remember that light travels at a finite speed. That means that in the fourteen billion years since the universe was born, light has only been able to travel fourteen billion light years. So when we point our telescopes into the night sky, the very farthest things we're able to see are fourteen billion light years away. That's why we say that the size of our "observable universe" (that means the universe we're able to observe) is fourteen billion light years.

Now, there's more to the universe than just our observable universe, but we don't know what it's like because we can't see it! But in short, that's why the universe has no end - it keeps going and going, but we can't know anything about it farther out that fourteen billion light years because light from anything further away hasn't had the time to reach us.