Good question,   a) only ever bet what you can afford to lose       b) If considering betting on a sport you don't know or follow, Do some statistics research like recent games vs each other, home and away records, current streaks, injuries.
I have bet on sports like u.s. college football, boxing, ufc, this way, have won some and lost some.  I like taking underdogs in these games/sports if odds are right and data says outside shot of winning.