There are 2 main ways to gain exp, doing missions and fighting.

Missions:
To start a mission press the N key, a window will appear with Tyndal [Zion], Agent Gray [Machine] and Flood [Merovingian]. As a Zion operative only do missions for TYNDAL as she is the Zion contact.

If you do the other Organisations missions you will loose Zion reputation points which eventually will change you to either Machine or Merovingian. There are various types of missions the ones listed under CRITICAL are the current story line, the other missions are varied in their content.

For best EXP select HARD. I have found the fastest way to gain fast good exp is to do TINY HARD missions and if one is available level with a higher level preferably a level50.

Also ask in faction chat if anyone is doing missions in particular archives, this will be benifical to both you and the higher level players as you will lower the level of the enemies for them making it easier.

Fighting
By fighting gangs and enemies you gain exp. You DO NOT GET XP FROM DUELS OR PVP. An effective way to gain exp from enemies is to go to a hideout where the enemies are a similar level to yourself, where you will get good loot for your level as well as the EXP.

There are other ways to gain xp, although the first 2 are the main ways.

Other methods include:
Data Node tapping - you need the data miner tree

Access node tapping - you kill a Gang leader to get a key to a node, which will lower your chances of being attacked by an agent and also lower their level from 255 to 100 good exp can be gained here especially in Downtown, get a higher level to take you its possible they havent tapped the nodes either so it is advantageous to both of you

Farming Robot Parts in 01 - In 01 the lvl 40-50 construct you can loot robot parts which can then be traded with Binary boy for EXP He is located in Downtown in Creston South Central next to Ouroboros HQ. players lower than lvl 40 cannot enter 01 however if you are on a team with players that can enter the construct you can still receive the loot, bare in mind the exp gained from the parts is scaled to your level, it is probably best to use this tactic when you reach level 40 as you can farm yourself.

Hardlines: At early levels it is recommended to tag hardlines, Richland alone will get you from level 2 to level 10, If you wish to hit hardlines in higher danger areas like Downtown, westview and International, it's recommended that you go with a level 40+ with the ability to teleport you.

When you first begin in the Matrix Online, a good way to level up is to collect Chopper Keys which can be traded in for both clothing from special collectors and more inportantly for XP. Trade them in at a collector called Codebase near the Uriah SW Hardline. However this is only a viable way to level up until you reach level 7. After that you should head over to the Mars Industrial Storage to do the Headless Quest.

XP can also be obtained from trading in Decaying Code at the Fumigator near Bathary Row.