Well, an army would be ground/land forces, as opposed to a navy, while military is usually a catch all term for both. But, “when” is critically important here. The history of armed conflict in human history is so varied and scattered that without knowing when or where, you’re really not asking a question that can be answered. I’m sorry.
You can start with a world history text, or Wikipedia if you want a time frame to start with. For that matter, Wikipedia is a pretty decent research primer these days. Just, remember to actually check other sources before you accept something as fact.
If it’s a fantasy setting, then asking yourself what setting (or settings) inspired you, and researching what pieces of history they used could be helpful. Also, role playing games with well fleshed out settings, like D&D’s Forgotten Realms (or Dragonlance, or Dark Sun, or Planescape, or…) and White Wolf’s Exalted can provide an absolute ton of world building to work with. Even just trolling a wiki for games like The Elder Scrolls or Kingdoms of Amalur can offer you some insights into world building. And of course, if you’re writing fantasy, read some of Robert E. Howard’s Conan stories, no, seriously, read them.
Also, once you’ve got a time frame in mind, the military history section of any convenient bookstore should have some good resources to work with, even if it’s not 100% applicable, you’ll learn a lot from there.