How important is one's first publication in affecting prospects, assuming others are to follow? With respect to a first publication, how would the following factors affect prospects (in academia, as well as in research-based industry)?


Whether or not the publication is in a top-tier journal. Of course, I realize that the better a journal, the better. What I mean to say is, given that I have some work now (such as, for example, observing certain trends when a simulation is run on a high performance computing cluster, and explaining any anomalies that arise) which may be publishable in a non-top tier journal, and some other work (which works towards providing improvements to a recent paper) which has a chance of faring better than a non-top tier journal, but which will take more time to get ready (say, 6 months more?), is it worth waiting for the completion of better paper (i.e., the paper which may get into a better journal)?
Like in 1, given a choice between publishing as a non-primary author now, versus publishing as a primary author later, should I wait to publish as a primary author later, and only then submit the journal in which I am not to be a primary author (assuming, of course, that the primary author has no issues with waiting).
Would it matter if one's first paper is in a journal or a conference? I have heard that it is better to send a paper in for a conference before a journal (especially for a first paper), since conferences tend to give much quicker replies. Am I correct?
Does the content of a first paper matter? Whether it is a review-based paper, a paper suggesting improvements to an already existing paper, or a paper submitting a reasonably novel idea? Assuming of course, that other methods or ideas would eventually be published, should I, say, wait to propose something more, umm... novelish first?


Long story short, how much would my first publication (in terms of parameters such as reputation of journal, how much it goes towards improving knowledge in the field, whether or not I hold primary authorship and so on) affect my prospects (whether in the industry, or in the academia).

NOTE 1: If it matters, this question is with specific reference to Computer Science (a little more to the applied side than to the theoretical side). Also, I am an undergraduate.
NOTE 2: By a journal of less repute/non-top tier journal, I do not mean to say a disreputable journal, such as a predatory one, merely a journal with lower impact factors, higher rates of acceptance, et. al.
A first publication matters less than you might think, especially as an undergraduate.  My take on likely interpretations:


If you publish as first author in a credible but low impact venue (of any type), people will think you're doing great for an undergraduate to be publishing as first author at all.
If you publish as first author in a high impact venue (of any type), people will probably assume that the senior author on the paper did the critical intellectual work in defining a high-impact research plan, and that you just executed it (whether that rightly or wrongly describes your particular situation, that is still what is likely to be assumed, given the difficulty of publishing in high impact venues).
If you publish not as first author (in any venue), people thing you're doing good work as an undergraduate on the team, but that the first author contributed more (which will probably be true).


Frankly, all of these sound like nice things to have people think about you.  My recommendation for any undergraduate is to not get hung up trying for a perfect first publication.  Your first publications will probably either involve a lot of hand-holding or else be a pretty rocky experience, as you gain experience in writing publications and communicating with your scientific community.  It's better to start writing and gaining that experience now, rather than saving it for a more painful experience with a larger and more significant work that you are more heavily invested in.