This is a Red-eared slider turtle. Please keep in a bigger tank.
Let me tell you these animals can grow big and very big !!!!
I had 4 RES and they were 12inch in size and that time I ran out of my money to buy a bigger tank for them.
I donated them to National Park RES enclosure.
First thing first,find a vet and it is very very important to locate one immediately. The baby RES are prone to many diseases. Not every vet is good with reptiles but if you are determined to get a good one then you may search one good vet first.
A big tank size is good for them and use a good structure to make a Sand and stone area. Where they will come and rest and can easily go back to water if they want to.
USe a big basking area and use a UV lamp and lit the lamp on regular interval.Make sure your tank has a secure cover and not open to any predators. Alternatively you can place it under sun but give them a natural bask but again be very careful on the natural predators like crow and cat.
This again can bring algae problem in your tank so the best approach is place them in a plastic tub and put water till their shell is covered and cover the tub with a porous cover and place it in sun.
You may also feed them in the same tub so it will save your tank water and leave them sometime in the water as they tend to poo in the same water after the food.They are messy eaters are loves to eat in water. So once the basking and eating and execretion is done then throw the tub water and dry it. Keep the RES immediately in the main tank.
Food can be little tricky to feed them as they will always prefer nonveg over veg. You also can use turtle sticks but don't feed them only with that easy option.
Use a proper heater and it is very important to them as without a proper temp control they slip into the hibernation and which is deadly and will kill your turtle.
For disease and food and other details careful on the Salmonella infection while handling them so wash your hand throughly and after holding one. If childern are handling them then make sure they are washing their hands off.