For at least half my life now I've favored military time, or the 24-hour clock, over the 12-hour clock which is much more common in the United States. When I was younger and learning how to convert the 24-hour clock into 12-hour time, I accidentally discovered an easy solution: Simply minus two from the hour, and then drop the first number.
For example, 18:30 (24-hour clock) = 18 - 2 = 16, drop the first number and we're left with 6. Therefore, 18:30 is 6:30 on a 12-hour clock! It gets slightly more tricky when the 24-hour clock gets to 22:00, because 22 - 2 = 20. But now you simply replace the leading 2 with a 1. Therefore, 22:00 = 10:00. Telling AM vs PM is easy: if the time is greater than or equal to 12, it's PM. Less than 12, it's AM.
Once you've used a 24-hour clock long enough, you won't need such methods of converting between the two types. For me, when I see the number 18 I now automatically see 6pm, and when I see the number 23 I automatically think 11pm. However, for those who are clueless when it comes to 24-hour clocks, this simple method of converting between the two might help. Of course, the only way you're really going to get used to reading military time is to change all your clocks (at least as many as you can) to use a 24-hour clock.