From The Urban Dead Wiki
In Urban Dead, a character's level is equal to the number of skills they have. A player's level is calculated from the total of both human and zombie skills; the only way to be above level 23 as a human or level 22 as a zombie is to purchase skills from the other side. Thus, the absolute maximum level obtainable within the game is 45. However, most survivors max out at level 43. To progress beyond this level requires obtaining the Brain Rot skill-which makes getting a revive significantly more difficult.
Earning enough XP to buy a new skill does not automatically cause the character's level to rise, only the actual purchase of the skill does so. Thus, many high-level players have hundreds or thousands of unspent XP. You can view your level (or others) by viewing their profiles.
The stats page has a Class Breakdown section which displays the number of players currently at each level. However, these figures have historically been somewhat misleading because two non-skill factors can also increase the levels of characters as they are reported there. The "flags" for having an infection and for undergoing revivification have both been treated as skills when reporting the number of players at each level in the past. That is why, at any given time, there may be several characters reported on the stats page with levels that exceed the maximum possible in the game.
New characters find trouble leveling up. The game, in all honesty, is hard at the beginning. Starting military characters, firefighters, and zombies can build up XP through combat; beginning characters in other classes can gain XP quickly by finding First Aid Kits and using them on wounded survivors (and zombies). Additionally, people find joining a group helps significantly, another way is to read the Guides on this wiki. In a group, you have more experienced players you can turn to for advice.