Answer:
Pick a side, i would chose civilian
put the date anywhere between Apr 12, 1861 – Apr 9, 1865
" Dear diary,
today is ________"
make a name, talk about living near a battle site, talk about what time of the day it is (morning, afternoon, evening, night, late night), where you are fighting (Richmond Virginia, Gettysburg Pennsylvania, etc.), last why you decide to join the army and why you are fighting the battle (my family was killed, 'i want justice', my parents forced me, i am their only hope etc.
You can do it. It only requires a little imagination.
p.s i just assumed you were talking about the Civil War