boyfriend left for army basic training

me and my boyfriend have been inseparable for over three years. when he decided to join the national guard last year, i can say i was not a fan. but as time went on obviously, i came around to support him. he left yesterday for army basic training and will be gone for 3 and a half-ish months (it really bothers me that i don’t know the exact dates). i got the call last night “i have made it to … you will receive a call sunday”🥲. does anybody have advice on how to get through it, how i should send letters, what to expect etc.