It appears to be a sapling of Common Buckthorn (Rhamnus cathartica), an invasive species that has established itself along farm hedgerows, and our backyards. You might have a whole bunch of little seedlings around this one, as the round, black berries are quite viable. Look around your property, perhaps one of your neighbours has allowed this tree to grow. Even so, the warty stems, caused by left over leaf scars are an indicator of this species, plus, if you see a sharp point at the end of some of the branches, this is what you have.
If not, then a little more information about what you see would be helpful.