People have some differing thoughts on what exactly 1st level PCs are in the game world and what distinguishes them from an average person. However helpful they may seem, level titles are big source of the confusion. Here is how I interpret 1st level PCs in the games I run.
Fighters
What differentiates 1st level fighters from your typical 0 level man-at-arms is that fighters survived a conflict where others like them perished. Could be anything from fighting in a war to fending off a pack of wolves. What's important is that fighters went through the crucible of lethal combat and emerged more alive than before. First level fighters are called VETERANS.
Magic-users
Beginning magic-users have studied in the ways of the sorcerers of ancient Zenon, where it is said all secrets of magic were known. Through rigorous cultivation, a magic-user's mind is prepared to serve as an intermediary between this world and the unseen realms from which magic is derived. First level magic-users are known as MEDIUMS.
Thieves
Anyone can slink in the dark and steal an exposed coin purse but a real thief is something more. To move in true silence, vanish in shadows, scale sheer surfaces without aid, intuit the precise workings of a complex mechanism and the vulnerabilities of an unsuspecting target—all are feats that exceed the capability of mundane folk. But it's hard to learn these things on your own, so it is assumed thieves start as inductees in the mysterious thieves guild or some other branch of the organized crime network. First level thieves are APPRENTICES.
Clerics
Clerics are closely tied to their alignment. Beyond conventional worshipers, clerics are the earthly champions of Law and Chaos, and are granted numinous powers for their devotion. While some clerics may follow a specific patron deity, all clerics of the same alignment are more or less part of the same faith. Newly initiated clerics must still prove their worth, and so are relegated to executing their function without the use of spells. First level clerics are called ACOLYTES.