M.H.
You don't and you can't. It is harder for some kids then others. My daughter was the ONLY one crying yesterday morning in the 1st grade. It rememinded me of Kindergarten. The best thing I can do is try and get her to talk about her fears. So far these are her fears: the long day (she was a 1/2 day Kinder), lunch - thought the teacher was going to make her eat her entire lunch that I packed and she can't see over the lunch cart so is afraid of getting lunch from school. She is also afraid of not understanding the teacher's directions and asking for help. She DOESN'T want to get into trouble.
I tried to bring out all the positives of school - recess, playing with her friends and learning new things - and made examples of what she learned last year and how much fun it is to now read and write notes, etc.
I let her sneak a few small friends into her backpack that stay in her backpack and are NOT allowed out. I took her school before it started and showed her the room and we met her teacher in the hall. I would have showed her, her desk and all but the room was locked. I give her little sayings that come on Dove Chocolates - which she loves. Right now she keeps saying the one "Smile when you want to, cry when you have to and laugh you can" - something like that.
Most importantly - you can't fake them out. They are scared little beings. My husband still remembers hating his first day. I don't. Each child will take different memories from their first day and apply them to their own life.
Just be there for your son, comfort him, tell him you'll be on time to pick him up and help him to talk.
Positively,
M.