Is it possible to kill Cicero before the DarkBrotherhood questline

He has 500 gold on him as well as his gear. Since you can pickpocket him and receive higher grade gear at the Dawnstar Sanctuary, it is really not lucrative to kill him. If you have completed the quest line for the Dark Brotherhood, and Cicerio is still alive at that point, he will indeed become a Follower.

Leaving him alive thus results in more benefits for you...

It sounds like you're asking about the first dragon you fight, outside of Whiterun, as part of the quest Dragon Rising: you follow Irileth outside Whiterun and upon completing the quest:

The Jarl makes you Thane.

If so, it's supposed to be an introductory dragonslaying encounter: if you followed Irileth around town before heading to the watchtower, you should have a fairly decent posse of guards to help you out.

To be honest, I was still learning how to use the controls on the PlayStation 3 version of the game at this point, and I essentially ran around the watchtower like a chicken with its head cut off. The guards and Irileth will do most of the work. I was a Conjuror build, so I summoned a Flame Atronach for good measure.

After a few minutes, the dragon will die pretty easily, whether you made a significant contribution to the fight or not.