I published in the top journal in our field (computer systems) when I was a junior undergrad. For me, here are some key factors that I think you need to publish as an undergrad:

1. Find an advisor that is willing to invest his time to help you grind towards your paper. To be honest, thinking about the trouble I brought to my advisor, I would be reluctant to advise an undergrad in the future. You need to find someone who is willing to go through that.

2. Read a lot of papers in the desired field of study. Read a lot. I mean seriously, start right now. Most papers cite about 20~50 other papers. Probably less than half of the papers read for that project ended up being cited. This means you need to read up to 100 papers to write just one. My paper had around 50 references.

3. You need to have at least 1~2 years left until graduation. Just submitting and waiting for the results take about 6 months in CS. You'll probably get rejected at first. That's a year, after you actually wrote the damn thing. In order to build the muscle needed to conduct research, you probably want to do something like a UGRP one or two years earlier.

So yes, a first author paper at a proper journal/conference during undergrad is quite a feat. Not just because of the skills, but because of the amount of luck you need. But it's definitely worth it, especially if you want to get into a good graduate program.