To get the brakes working, one needs a controller of some sort (search this group for lots of info), which must be installed and wired into the tow vehicle. Prices for the same unit vary, so do some homework -- EBay is a good source.
The brake wire needs to be extended to the trailer, so if you have only a four-pin connector (aka Flat-4), that will have to be changed out.
Most states require (and it's a good idea anyway) a break-away brake switch to apply the trailer brakes in the event it becomes separated from the tow vehicle while moving. This in turn requires a battery
on the trailer, either the house battery
or a special small battery
if no house battery.
Finally, the brake power wire will need to be connected to the electric brakes.
If you are handy, there are lots of folks willing to pitch in with advice on doing it. If not, ask for recommendations as to where to get it done.