If someone is having a first Communion, then you have to a Communion cake for this special occasion. There are many different ideas you could use for the cake, but the central point is to stress the religious significance of the event for all attending.
Bake two layers of a square cake, using the instructions on the package. The size of the cake depends on your needs. Let the layers cool for 20 minutes so they can dry a bit (meaning the surface is now stiff enough to add icing).
Cover the first layer of the cake with a smooth, white icing, then place the other layer on top and cover it as well.
Download a religious image such as a cross that you would like to use (see link in Resources). Then print out this image at the size you need for your cake. Tape it down on a table. Then tape wax paper over that. The design should be visible though the paper.
Start adding icing to the design, following the underlying shape. If there is more than one shape in the design, use different colors. You can also vary the amount of icing to make one object thicker than another. Place the completed design in the freezer for about 20 minutes.
Remove the design from the freezer and gently rub your finger on the back of the wax paper to loosen the design. Once it's loose, place it quickly on the cake.