Realize that being an Army girlfriend is much different than being a girlfriend to a civilian. Your relationship is restricted as many things will interfere with the time you get to spend with him.
Accept that there will be chunks of time where you won't be able to see or speak with him. Soldiers are often swept away to train in the field, or go elsewhere for weeks of training. You'll be lucky if you get to talk to him for five minutes a day, if that.
Learn to be independent. Should your soldier be deployed, you'll be without him for a year or more. If this is something you will have difficulty handling, find another man. Your boy will need your support while overseas, not your indecisiveness.
Your soldier will most likely be very close with his friends especially if he has been deployed. It's important that you support and encourage this. These guys are very close and will do almost anything for each other.
Expect him to be tired most of the time. Soldiers work long days. They often have strenuous Physical Training activities and work in the heat with several layers of clothing. There's nothing better than giving him a nice massage after work, or cooking him a nice dinner.
Get used to hearing words and acronyms that you won't understand.
Soldiers get up very early in the morning. Respect this and say goodnight early. If you are staying together, take the time to get up sometimes in the morning to make him coffee.
Realize that the Army has to come first in his life, not you. It's not a normal job. He can't be late for work, he can't leave work if you need something and it may be hard for him to keep in touch with you during the day.
If your soldier is staying in the barracks, understand that there are rules. There are typically no visitors allowed past 10:00 (that's 22:00!), he will probably have a roommate and you won't get to spend much alone time with him when you're visiting.
Be proud of your man! What he's doing is not easy. The best thing you can do is be supportive, listen to him talk about his day and love him as best you can.