It could be a red-eared slider or a yellow-bellied slider. At that age they look pretty similar. Could also be a cooter. If you can get it to extend its head it will be easier to tell. The carapace coloration looks a bit more like YBS than RES to me, but there's a lot of variation and some hybridization possible.
Are you planning to keep it? If so, you may want to check out
https://www.myturtlecam.com and
http://www.austinsturtlepage.com first so you know what you're getting into. The start-up expenses and amount of work required are considerably more than is required for most other pets.
Also, you'd have to set up a uncycled habitat, jump start it with something like Tetra Safe-Start and/or Seachem Prime, and monitor it very closely until the nitrification process is fully established (usually between six weeks and three months). High ammonia or nitrite levels will kill it, and high nitrate levels will make it very uncomfortable (and possibly kill it).
Female YBS and RES can grow to a length of more than a foot, and the habitat should have at least 10 gallons of water per inch of carapace length. So you're looking at an immediate 20 to 30 gallon tank and an eventual 120 to 150 gallon tank, depending on how you mount the basking area. The water requirement refers to the amount of actual water, not the tank size; so if you mount the basking area inside the tank, you lose the top five or six inches of space when it's an adult.
Turtles in captivity also need a light source that mimics the sun in color, intensity, warmth, and both UVA and UVB light. This is a life-or-death thing for them. Most captive turtles kept by beginners die from metabolic bone disease due to improper lighting. (Poor water quality, improper temperature regulation, and overfeeding are the next most common causes.)
Don't bother trying to feed it out of water. It can't eat out of the water. Water turtles don't produce saliva.
On the positive side, once a proper turtle habitat is fully-established, they're relatively hardy, moderate-maintenance pets. They also can be left alone for a few days if need be because they're opportunistic feeders, so they can go a few days without feeding. In fact, the most common feeding schedule for adult aquatic turtles is every other day.
Rich