how does one decide to probably do something?

Anyways, portfolios primarily aim, when applying from a non-design field, to showcase how to think and resolve ideas using the very basic design elements (light, shadow, scale, shape, perspective, etc).  This can be achieved numerous ways in all sorts of graphic or sculptural mediums.  Don't bother trying to show architecture as it is understood/expected that your, respectfully, don't know shit about it.  

So, what can you do?  Start by sketching, sketching, sketching.  Draw things around you, quickly, and work on getting ideas and visions onto paper.  Don't rely on photographs either.  Go somewhere you find interesting and get some scenes down and take it from there.  A few evening life drawing classes will certainly help too.

Find some architects to have lunch with and pick their brain about what the job is like.  Better yet, find someone to shadow to get an idea about what the day-to-day looks like.  You may change your mind very quickly.  Since you're a senior - maybe you have one semester left? - if you are at a school with an architecture program, see if you can take a few intro classes at your university.  Or even just audit them and see if you like them.  (I changed majors into arch after sitting in a few architectural history survey courses.) 

I, too, am curious how this decision came about.