So, if you purchase a baby turtle that's under four inches of shell length, do realize that that is against FDA regulations. The baby turtles are more likely to be handled by young children and there's a risk for young children to contract salmonella from he baby turtles and from the environment of the baby turtles. Salmonella is a bacteria that causes illness, diarrhea, abdominal cramps and vomiting in people. If you end up with a baby turtle anyway, essentially the way you would take care of it is similar to how you would care for an adult of that species with some minor differences.
The first thing you want to do is actually research the natural environment of that species of turtle. So, keeping in mind that turtles can be terrestrial, or living on land. They can be aquatic, or they can be both, aquatic and living on land. So, the goal to proper turtle care is always to mimic the natural environment as best that we can. Baby turtles, unlike their adult counterparts, are often much smaller in size and a little bit more shy. Again, they have more predators in the wild, so you may want to set up your enclosure to include more hiding places. The diet of a baby turtle can be different than the adult. They're growing more rapidly, often have a higher need for protein and other nutrients, so again you need to look up what the diet is of that species in its natural environment and do your best to replicate that.