I do have a site that could help you, however, I will need to get a moderators permission before posting it, the site does contain swearing, inappropriate things, but the articles are all great. Shows where all skulls are, easter eggs, ect.
To customize your character like your friend, most will come from Online play.
However, you can get armor from doing the following.
Collect 6 skulls - Shoulders
Collect 9 skulls - Body piece
Collect all skulls - Helmet
Beat Tsavo Highway on Legendary - Shoulders
Beat The Ark on legendary - Body piece
Beat game on legendary - Helmet
Using the tank is probably always the safest way ;P