i didn't customize my appearance :(

Iarao

may I do it now that I've begun playing the game?
is there a guide for how to do things like view inventory, etc?

LoreRunner

You cannot change your character appearance once you are in game at this time. You will have to create a new character (do not delete your old one until you have all the mail placed in the bank though or you could lose it).

For ingame tutorials, there should be pop-ups when you first run through cold harbor either directly in front of your face or to the bottom right tell you how to do things. Use F1 to get to the help menu otherwise and it provides all the details.