Judgement, Head on over to donedealpro.com and check out the forums. There are quite a few threads on how to hunt down an agent. Asking for one in the DVXforum will not get you an agent, nor a referral.
Then, do a little research on the web, there's thousands of resources you can read to educate yourself on the process.
You do not need to "establish yourself" first when trying to secure representation. Nor do you need a sale under your belt. What you do need is an excellent, marketable script. Just do a little research on query letters and insure your script is polished. This means re-write and re-write again until have a quality product. If you have an excellent query letter and an even better script you'll have more opportunity than an established writer with 3 sales and a crappy script.