One thing to consider first before having an outline of a turtle's enclosure is its natural habitat. Some turtles, like softshell turtles, have evolved to be burrowers and should be supplied with a fine substrate that encourages their natural behaviour. Others may live in muddy environments, and make use of that, but do not need it to thrive (like red ear sliders in some habitats.) One thing to point out is the use of small gravel, many turtles swallow them, and though they'll probably be pooped out, many can cause impaction (a blockage in the intestines).
Most turtle hatchlings will prefer shallower habitats, that can be provided, but it's advisable to never let the depth lower that the turtle's shell width. With time baby turtles will develop stronger limbs with a larger surface area of webbing, making them far better swimmers (and harder to handle). As long as the tank is wide and spacious, the depth of the water can be kept near maximum for most species (ofcourse while making sure the turtle can't escape.) Also be sure to provide supports near the water's surface so the turtle doesn't fatigue trying to break it for a breath. A general rule of thumb is 10 gallons per inch (around 15 liters per cm) of turtle shell, but since depth isn't direly important to turtles (as long as it's enough), a surface area rule of thumb is probably more suitable: 8-10 times the length of the shell in length, and 4-7 times that in width.
Being cold blooded, turtles require a heat lamp to thermoregulate on a dry portion of the enclosure, as well as an appropriate water temperature. They also require a UV-B lamp to aid them in the absorption of calcium by causing vitamin D3 synthesis in their bodies. The UV rays will be blocked out by a plastic or glass cover, so be sure to get a screen cover, or clamp the lamps, but be sure that they are safely clamped as they are a fire hazard. Your dry land can be a typical commercial ramp or dock, driftwood, rocks that aren't sharp (make sure they won't topple if you're stacking them), or a custom made acrylic dock. You will need a thermostat to keep the water between 74-80 °F (23-27 °C) for most species, and the basking temp should be between 88-95 °F (31-35 °C).
Lastly, there is filtration. Sponge filters typically are ineffective biological filters and don't do well with turtles. Your best choice would be a canister filter, but any filter with good mechanical filtration (multiple filter pads), and appropriate bio media (bio balls, ceramic rings, porous objects, etc.), should do. You want to aim at cycling the tank three times an hour, so if you have 10 gallons (37.8 liters) of water, you need a filter that cycles 30 gallons in one hour. You should make sure your water is free of chlorine and chloramines, and if it isn't use a conditioner like reptisafe.
Other than that, you can add some plastic plants, especially ones at the surface to provide support and make hatchlings feel safer. Some turtles have been known to bite at fake plants so be sure to watch your turtle for this habit. You can also add live ones, but turtles can make a real mess out of them so it's preferable to grow them separately then add a few at a time.
