Difficulty getting published in top tier journals?

I've co-authored a paper on a top-journal. The first author was a PhD student. The paper was so strong that it was even on the cover page of the journal.

That was the first paper for both the first author and myself.

But ...

• The paper was authorised very closely with some experts in the field. Those experts knew exactly what to write, what to analyze and what not to do. It's a domain knowledge that a student wouldn't possibly have. They knew how to conduct a good research project.
• We wouldn't been able to publish to the journal without the experts.
• The study was indeed interesting, and attempted to solve some very practical problems. Thus, it was not something you read and throw away. It would cause impacts to the field.
• We had softwares, web sites, cool graphics to backup the paper.
• There was nothing special in the statistics and methods. Absolutely nothing new. But how it was analyzed was simple to follow and solid. Again, the experts knew exactly how and what to do.

You have better chance if your study attempts to solve important questions. Talk to a professor if you need advice.

Unfortunately I don't see you have much chance if you don't have anything truly interesting.

How much harder is it to get published in a top-tier journal (ACM, IEEE) compared to a small second-rate conference?

Depends on the journal and conference. For example, ACM SIGCOMM/MOBICOM will be much harder to get in than a lot of the top journals in IEEE/ACM. By the way, a lot of Editor in Chief writes a 'summary' of how their journal has gone for a given year. He/she usually gives you the acceptance ratio and you'll find that it is usually higher than conferences.

Is it an unrealistic goal to try to publish my 1st paper in the #1 journal in the field?

Perhaps, but this very much depends on the area and who you have on your team. Like @SmallChess said, you need experienced co-authors. They will help you pose the right problem, advise you on the methodology that is considered the norm in community and presentation.

Usually, a junior researcher will not tackle a significant problem. This is due to the lack of knowledge and also he/she doesn't have many tools to mount a good attack on a significant problem. Consequently, his/her contribution will be small and thus makes it harder to publish in top places. Having said that, there are 'junior researchers' who are the top of their field. Then they are no different to a seasoned researcher.

Is it likely such a journal will accept my paper if I am proposing a hybrid method that combines methods used by other papers? Or do I need something completely new and ground-breaking?

Maybe yes or no. Does it yield a surprising result? It's about contributions. If your contributions are not significant, then maybe no. So think about how significant your contributions are before trying.

Does the fact that the 1st author will be a student (undergraduate) will reduce my chances?

People always assume the first author is a student. So they look at the second, or third author. :)

In answer to your query about whether or not it is an unrealistic goal - not necessarily, you won't know unless you try - but chances are, you'll have to do more work (research, revising etc) for the journal. Having said that, consider the following:

• as with any manuscript submission, you need to fully read the 'Aims and Scope' of the journal to see if your work matches.
• Read through some of the recent articles of the journal to see the style and the depth of the articles that get published.
• Do a search within the articles of the target journal to see if your research has been touched on before.

If in doubt, send an email inquiry.

If possible, have an advisor/professor help you out (perhaps as co-author) - their experience could very likely prove to be invaluable.