It will be difficult to get a puppy from a reputable breeder or a dog from rescue if you do not have an adequately fenced yard. Boxers are, by and large, fabulous jumpers, and even many females can clear a five-foot fence from a sit with little or no difficulty. You therefore want a six-foot-high fence, minimum. You may also want to make sure that there is a barrier beneath the fence line to keep your boxer from digging his way out.
The fence needs to be strong and free of cracks, gaps, or exposed nails. A solid wood fence is preferable to chain link or any other type that passing people and dogs can see through. In general, you want your fence to serve as a visual barrier as much as a physical one, as other dogs may frequently pass by, possibly riling your dog into a frenzy.
Invisible fencing consists of an underground barrier with a battery connection that the dog wears on a special collar. The collar gives the dog a shock as he crosses the barrier, reminding him of his boundaries. Invisible fencing is generally much cheaper than barrier fencing or hurricane fencing. However, boxers have a very high pain threshold, and they may not care that they are being shocked. Furthermore, while this method may keep your boxer in, it won't keep other dogs out.
Beyond putting an adequate fence system in place, you also need to make sure that your yard is free of any items that may harm your dog. This means that all potentially toxic plants (primarily flowering bulbs) must be removed from areas of the yard where the dog will be permitted. Grass is probably the best medium for most dogs, with a nice large tree or two for shade.
If you have decided that you want your boxer to potty in only one part of the yard, you should cordon that part off, or otherwise define it in some way. Cover this area with the kind of mulch or ground cover that is appropriate for your geographical area. To teach the dog that this is his potty space, march him out there as soon as he wakes up from a nap and periodically during play or lessons. When he does his duty in the right spot, be sure to give plenty of praise.