Without going into too many details that would be of the spoilery nature, Prey has a nice cache of supplies at the very beginning of the game. If you’ve already started playing the game you may have already passed up this safe in Simulation Debriefing Room, immediately on your right when you exit your bedroom for the second time.
If you do go in there initially, you’ll notice on the chalk board that there’s a code smeared out on a whiteboard. The code used to be there, but it can be found a little bit later on with a little curiosity.
A couple hours into the game you’ll make it to your office, where you’ll watch a Looking Glass video of yourself. If you’re looking closely, the room that the video is being recorded in should look familiar, it’s actually the Debriefing Room and the code that is smeared in the present, is legible in the past. Looking to the left while watching this video will reveal the code of the Debriefing Safe to be 5150. Unless you know the code before starting out, you’ll need to backtrack a bit. Sure, you can hack the safe with Level III Hacking ability, but if you want the contents in it fairly early on this is the fastest way to do so. Be careful though, as you go back towards the Debriefing area, you’ll encounter numerous Typhon enemies.
The contents are worthwhile. You’ll get a Hypo Psi Fabrication Plan, a few Psi Hypos, 600x Exotic Materials for crafting, and a powerful upgrade for your scope with the [O] Pistol Critical ++ (which increases the critical chance rate when firing pistols).
Update: After going back and playing the opening sequence to see if you could unlock the safe without having seen the message on the wall it appears that the upgrade chip that you get will be for whatever weapon you are holding. So if you do open this safe in the early game it appears that it’ll be an upgrade for the wrench. You might want to wait until you have a more powerful weapon like the Q-Beam.
You can check out the video below for the location of the safe.