Prune approximately 10 percent of the limbs each winter to increase sunlight to the center of the tree and to encourage new growth, which in turn will produce more fruit. Cut damaged, older and the least-productive stems back from where they originate, using pruning shears, loppers or a pruning saw. Thin crowded areas as well. Evenly distribute pruned branches to maintain a nicely shaped tree.
In your case ~10% at present seems to mean at most two (whole) branches. The level at which low branches near the trunk will be an obstruction will rise with age but since planted in grass you presumably will be mowing and so may appreciate reasonable headroom, so might want to end up with about 4' of bare trunk. Unless ease of protecting the fruit from birds and harvesting is more important for you (assuming you do not have access to the equipment Woodyatt Cherry Farms has!) in which case you would presumably prefer the canopy as low as possible – see for instance How do I prune a cherry tree to keep it short?.
For timing, Stark Bro's advises (about pruning in general):
Plan to prune your fruit trees during every dormant season. In Zone 6 and farther north, you should wait until late winter.
though also mentions You can have confidence in knowing that not everyone will prune the exact same way (including the “experts”) and with their video (which shows some relevant practical details, like aiming to allow light and air into the centre) offer late winter and early spring, while they are dormant and before their buds break. Pruning is never normally a good idea in a growing season as when the tree is most active the flow of sap is greater, so more energy to waste from leaks and more to attract pests.