Use Item Shortcuts
You can favorite items. And once you favorite your Stimpack or favorite guns, you'll be able to quickly access them using the D-Pad (if you're using a controller).
Deciding what to put here and where has been extremely important for me early on. I've survived numerous battles and encounters because I was able to quickly pull out the right gun or aid using the shortcut menu.
Managing this menu is going to be crucial to your success early on and you'll want to lay it out in a way that you can remember.