How and when to prune depends what you have in mind for the trees.
If you will be happy to have small bonsai with thin trunks and not much structure you can start trimming now. The usual recommendation is to cut off all the bits that are not bonsai and you'll be left with bonsai. In more detail, cut long shoots back to 2 or 3 leaves because that will force new shoots to grow from the leaf axils (base of leaf stalk). Over time that will give you bushier branches.
Also cut off any shoots growing straight up, especially if they compete with your main trunk.
If you would like a bonsai with a thicker, older looking trunk it will take forever if you keep the trees in little pots and trim regularly. Thick trunks come from growth. We regularly put our starter trees in larger pots or even in the garden and let them grow a lot to get thicker trunk much quicker. Then chop the trunk short and wait for new shoots to grow the next section of the trunk. Many good bonsai have been grown from the lowest part of the original trunk and the first branch as the next section of the trunk.
We can add more if you can let us know which way you'd like to go with these trees. It is also worth adding a location to your profile because much of what we do is tied to seasons and climate. Knowing where you are lets us give more accurate help suited to your particular area.