Shift it to the gear you like, then remove the shift lever and tie off the cable.
I'm not (entirely) kidding. A more sophisticated effort will not be worth the time and expense, and that would let you decide if you actually like single speed, in which case you should get a better bike. A small amount of effort ought to find a better candidate for not much money.
That bike probably does have a thread-on freewheel, if that's what you're asking, but it doesn't appear to have horizontal dropouts so you'll need a chain tensioner, e.g. that rear deraileur.
Personally, while I'm a big fixed gear and single speed fan, I go for gears pulling the kid trailer.