How good of a conference can I realistically submit my first paper to? How can I figure out which conferences I'm good enough to submit to? I'm working on finishing up my thesis in wireless security and I'd like to publish the results of my work in a conference, obviously the best conference that I can realistically get published in.

However I'm having a hard time figuring out what the criteria might be for this. I see conference rankings around, but how can you actually figure out where on that ranking you should fit? How can I learn how to evaluate these kinds of things?
You learn to evaluate these kind of things by experience.  You read the papers published in the conferences, form your own opinion of their quality, see how they impact the field over time, see how they are viewed by others, and revise your opinions.  Eventually you get a general sense of how "good" a paper should be in order to be published in this conference, and you compare it with your evaluation of your own paper's quality.

You bootstrap the process by drawing on your advisor's experience.

So: Ask your advisor.