Fewer erections will probably be the first thing you notice. You'll probably lose the perpetual urge to masturbate. Spironolactone is a diuretic, so be sure to drink lots and lots of water. For me, the first week was a battle to stave off headaches from dehydration. This will pass, but keep drinking water! Also, be sure to masturbate regularly, at least twice a week. Keep those kiegel muscles in good shape, and your orgasms will get really good.
After a month on spiro, I had girl nipples. I don't think this is the norm, though.
Edit: Congratulations on taking this big step!