How can I start this quest?

I watched some videos, and i believe she can be found in the Goodsprings Saloon, behind or near the bar.

-edit- by the way, it is best to ask questions like this in the spoilers forum.

x0563511 kindly points to the http://www.gamesas.com/index.php?/forum/108-fallout-new-vegas-cheats-hints-and-spoilers/ forum.

If she's not in the saloon, then try her house (particularly if it's night time). If I remember correctly, it's the white one directly across from the saloon and general store.

Ok found'er. I just had to do the toturial first. Now where is Cobb?

Oh, that's great. I have to do the tutorial to make her appear? I was concerned that it was bugged and some characters had disappeared.