why did you get a turtle without researching on how to properly take care of it!!! That is an extremely irresponsible thing to do!!! How big is the tank the turtle is in? It needs to be at least 50 gallons for the turtle to be happy. Turtles need a large basking area, they can not just have a piece of wood! The turtle will drown if you don't have a large basking area. You need to get a heat lamp, and a uvb lamp. Turtles bask under a heat lamp to warm themselves up. the heat bulb should be 75 watts: http://pet.imageg.net/graphics/product_images/pPETS-3756841t400.jpg and you need to screw that into the clamp lamp: http://pet.imageg.net/graphics/product_images/pPETS-3763570t400.jpg
thats why i am here to tell you what they need in case you thought they were easy pets....well you were wrong. Turtles are a huge commitment, and can live 30+ years. Red eared sliders are some of the biggest species with females growing to a whoppin' 12 inches and requiring a large 120 gallon turtle pond. Males are smaller and grow to 5-8 inches as adults. If you happen to have a male, then you get get away with keeping him in a 75 gallon for life, but 75 gallons AT LEAST for a full grown male, and 100 gallons at least for a full grown female.
if your turtle is a baby turtle under 2 inches it can be housed in a ten gallon, but only for a short time, 6 months or less. the setup should have a heater, filter, and a basking dock in the water. on top of the tank, you need two separate light/ heat fixtures. you need a heat lamp on top of a screen shining on to the basking dock making the temps 85-95F <check with a thermometer> and you need a uvb light, which is completely the opposite offering no heat, but uvb rays which are needed to be processed by the turtle to create the proper calcium and d3 they need to build a healthy shell.
here is the baby setup for 6 months:http://timothyrobert.com/trash/tank.jpg
You also need a mostly pellet diet filled with calcium and the vitamins they need, you need to get pellets especially made for baby turtles, because they are formulated with what babies need to grow, lots of calcium and vitamins. Zoomed hatchling aquatic turtle formula is great. http://ecx.images-amazon.com/images/I/41w9avh1BPL._SL500_AA280_.jpg Also feed Reptomin as the staple, it is very good for them. http://ecx.images-amazon.com/images/I/312PVPTE9YL._SL500_AA200_.jpg After the first six months you will have to cut back on pellets to 50% and introduce insects and live feeders like minnows and guppies into the diet. Insects include crickets, meal worms, snails, earth worms, shrimp and krill too. http://www.drsfostersmith.com/images/Categoryimages/normal/p-261576-42502-reptile.jpg These "prey" foods should be dusted with a calcium supplement like rep-cal to be nutritious. http://www.beardeddragoncaresheet.org/images/repcal-calcium.jpg Then after a full year has passed start slowly introducing dark leafy greens into the diet, especially M.C.G.R > mustard greens, collard greens, green leaf lettuce and romaine lettuce. As baby turtles grow they will naturally convert into a omnivourous diet, so they might reject it at first, but by the time they are adults, vegetation should be as high as 80% and protein sources limited to as low as 20%, so convert gradually.
so after 1 year has passed the turtle should have outgrown the enclosure, and be ready for the next, much bigger one, a 50 gallon tank. You need one this big because the turtle will have lots of room to grow and be healthy so its practically "the growing tank" small heaters and filters for a 10-20 gallon would certainly not be suitable for a 50 gallon so make sure that you would get a filter rated 3 times the turtle size (for 150 gal) since turtles crap loads, and you would need a strong filter to keep the water clean, and crap-free. And you would also have to upgrade to a better heater, a 200 watt.
"growing tank" - http://i27.tinypic.com/xbcv4o.jpg
recap / brands i recommend
Reptomin
Zoomed hatchling aquatic turtle formula
Reptisun 5.0
Zoomed basking bulb - 50 watt, 4 inches away
Zoomed turtle dock
Rep-cal calcium powder with d3
> dust the insects
Zoomed can o' insects series
RENA filstar xp3 filter ( bigger tank)
Visitherm Stealth 200 watt heater by Marineland
cuttlebones
doctor turtle sulfa block
thermometers
Good Luck and do the research please, turtles are not easy pets and do require lots of work and maintenance to keep them healthy. ATP has everything in greater detail:http://www.austinsturtlepage.com/Care/housing.htm and http://www.austinsturtlepage.com/Care/care.htm