[Gerrard]

Right, I'll try to build on what Allen said without confusing you.

The important thing with agents is your Standings under your Personal Info page. You'll notice in the "Liked By" tab a list of factions and corps with numbers by them. Your standings with a corp (and indeed faction if that's higher) will determine which level and quality of agents you can work for.
Quality effects how much money and LP you get when doing missions. It might also effect how much your standing goes up, not sure on that though. It does not effect the difficulty of the mission itself.
The higher the level and quality of the agent, the higher the standing you need. There is a formula, but it's just easier to demonstrate with actual numbers: -
Lvl1, Q-20 = -1.0
Lvl1, Q0 = 0.0
Lvl1, Q10 = 0.5
Lvl1, Q20 = 1.0
Lvl2, Q-20 = 1.0
Lvl2, Q0 = 2.0
Lvl2, Q20 = 3.0
Lvl3, Q-20 = 3.0
and so on upto lvl5 missions, which are for groups.
A higher level, but lower quality agent is always better. Ie, at 1.0 you are better doing low lvl2's than high lvl1s.
Finally, after every 16 missions you do for an agent you get a storyline mission from a different agent. It's very important you do these. Kill missions will normally be harder, but the reward is higher than the normal missions. Courier missions will normally have an attribute implant as a bonus, kill missions a combat implant. Most importantly though, completing it gives a massive boost to your standings with the corp and is the only type of missions that affects faction standing as well.
You can look for agents in the starmap. Can't remember the exact menu it's under, but I think it's under "My Information"/"My Agents". When you select it, you should get lots of green dots. The bigger the the dot, the higher the level of agent available to you. Hovering over the system gives you a list as of the agents and stations as well.