Hi Betty,
I've found two ways that have worked for me.
1. If you have a big serrated knife, you can just cut through the cake with the knife. The easiest way to do this is by placing the cake between two blocks (anything -- wood sticks, plastic containers, etc...) of equal height that come ~half way up the cake and resting the knife on those blocks while you cut through the cake. The blocks keep the knife's level even and results in even layers. You can also do this without blocks. It is, however, more prone to error.
2. You can use floss. Begin by lightly cutting the side of the cake all around it's circumference. Wrap some floss around the cake (using the cut you made as a guide) and where the ends of the floss meet, pull the ends sideways until it goes through the cake. For really dense cakes, this might not work, but I've had success with it.
Hope this helps!
Tim