How to come up with a millitary

If you want a strong military, you need to figure out which weapon type is being used by your enemies, your soon-to-be enemies, and your neighbors. If you have no enemies (not at war/hostile), check the ones where you're cool or wary, and if there are none of them, check your neighbors.

Determine which defenses (if any) they are researching.

You can determine this three ways:
+ Look at their ships
+ Attempt to trade with them and look at their weapons/defense techs
+ Look in the espionage/race info screen. Assuming you have enough espionage, you can find this information out.

Choose your weapon type based on what they are NOT researching for defense. Choose your defense type based on what they ARE researching for offense.

The best military is one that is custom made for the task at hand.

Shields don't do much for military. You'd be better off creating a fleet of small, mediocre ships, if you want military rating. If you want to actually fight, make the biggest, toughest, most survivable ship your technology allows, and get a few of them made, them back them up with cannon-fodder all attack ships.