It is, often, the very first discernible change. Within the first week for me (though I was then, the initial four months or so, on a ridiculously high dose - twice more than I ought to have been - so YMMV). And growth, I can attest, continues well beyond 2 years. True, not as quickly as the initial year or so, but it definitely continues.
There will likely be occasional... "soreness" I suppose (kind of like a really light bruise, best I can describe it), at least true for me, but nothing remotely unbearable. It should enlarge quite a lot in a relatively short span of time initially, so that's why.
But yes, DO find some comfortable, soft-fabric underwear (whatever the cut, doesn't really matter). I'm a boxer-brief's guy. I like Champion, basically "fancier" Hanes, best. Breezy, and a hell of a lot softer than their typical stuff at Walmart etc. all.
Otherwise, prepare to be 'inappropriately' aroused, often... a pressure-feeling (and indeed, it can get fairly uncomfortable), which might go away without "help" or may not... no 'cream' for that... and on that note, welcome to male adolescence. =P