Hello! So this is going to be a rather short chapter. Let's just get straight into it. The question to be had here is, how do you structure your military and how do you make it realistic? There are two questions to be asked and answered to accomplish this. Especially if you want something realistic and immersive. So lets have a look shall we?
How do I know what kind of military structure to use?: This question is by far the easiest one. If you look at real world history, the farther back in time you go, the less organized and thought out battles became. So, if your story takes place amidst a clan of peoples spread out over a small or large area, it is not likely you will have a very structured system. At best, the clan might have their dedicated warriors and warrior leaders, but that tends to be about it.
Where as a more advanced say Medieval or later period civilization is going to start to show a more complex military structure. The more advanced the civ and the more advanced their technology, the more advanced their military becomes to adapt to and confront the changes around them. So, the complexity of your setup really comes down to what kind of civilization are you building this for.
How should I structure my military and how do I give ranks proper titles?: The BEST advise I can give you here is to pick a real civilization that closely resembles yours in culture and then base your military structure off of theirs. And you don't have to use the same titles. You can rename them as you see fit, and if you can break outside the box here and use titles more representative if your civilization rather than generic titles like commander, captain, or officer.
That is really how easy it is. Now, you can always build one from scratch on your own in which case I stress again that the complexity of the command structure and division structure should reflect the civilizations advancement in technology. The more advanced, the more complex. And if you are working in a SCI-FI era, look towards games as a source of inspiration.
NOTICE: As always, keep extra copies of your work saved and accessable to you. My recommendation is to have a copy on your Hard Drive, a dedicated flash drive and a printed copy. Remember to name and date ALL your documents. Good luck!