One of the simplest ways to expand your organ composition is to add a contrasting middle section. By contrasting I mean it should be different in one of the several most important musical elements: thematic material, keys, rhythms, harmony, texture, and registration.
So you can choose a different theme for your second section, a different type of rhythm, contrasting keys or different kind of chords. You can also use less or more voices and you can use different stop combinations.
Let's call this middle episode section B. Remember your original piece will be named section A. So if you compose section B from scratch, then you will have two contrasting sections or episodes - sections A and B.
In order to make your composition complete you can repeat this section A at the end. The first section A then will be called an exposition and B will be called a recapitulation. In order to make the B section more complete you can design it in the same way as you have composed the exposition.
In other words, it can also have 24 measures. These 24 measures can be divided into three parts each of eight measures long. If you want your section B to be more interesting, you can change the meter.
In addition, you can also make your melodies much more different than in section A. For example, if in section A there were many leaps, then in section B the melody could be composed more in a stepwise manner. Similarly if in exposition your melodic material included more steps than leaps, then in this middle section you can do the opposite.
Try to apply my tips in your organ composition and create this section B with recapitulation at the end. This way you will have a nice ternary ABA form.