There are two versions of the verb.
"Å bestemme seg" is perhaps most easily understood as "to make up one's mind" or "to [come to/arrive at] a decision", but is in many cases translated as a simple "to decide".
"Å bestemme" is used as in the Norwegian sentence above, meaning "to make decisions", "to decide".
It's because it's a nasal+ plosive cluster: http://seas3.elte.hu/phono/notes/nasalplosiveclusters.html
Think in English "dreamt". There is no "p" but because of the plosive sound it makes a "p" sound.