Okay. I am guessing you want a Red Eared Slider or Yellow Bellied Slider or something similar here.
Choosing the right tank:
The rule for tank size is a minimum of 10 gallons per 1 inch of shell length of the turtle. But I would start of with at least a 20 gallon tank not a 10 gallon tank. So basically for example, if you have a 3 inch turtle you should be keeping it in at least a 30 gallon tank. But try to get the biggest tank possible. Female terrapins can grow up to an average of 12 inches in length and the male up to 10 inches. So in the future be prepared to have a 100 - 120 gallon tank or small pond of sorts. If you do not have sufficient money for this or are on a tight budget I would recommend a stock tank or large Rubbermaid container with a PVC pipe or wooden frame to keep it from bowing due to water weight
Heating:
You do need a heater in your tank. The temperature in the water should be at about 75-80F
Filtration:
Turtles are very messy eaters and big poopers so filtration is very important in a turtle tank. The filter should be of a good brand. But don't get the filter that is recommended for your tank size always go double that size for turtles
For example, I have my 3 inch RES in a 55 gallon tank and the filter is an Aquaclear 110. This filter is for tank up to 110 gallons and filters 500 gph. It isn't the best but it does its job
Best of all it cost me only $30 at Big Al's Online.
Basking and Lighting:
Turtles need a dry piece of land on which they can completely come out of the water and dry themselves. I have a Large Basking Platform from ZooMed. Over this basking platform you nned a very special lighting. First you need a UVA/UVB light. This provides your turtle with artificial sunlight which it needs to develop its shell. You also need a heat lamp (or "basking lamp") over this basking area that should keep the basking platform warm and dry. The basking area should always be 10 degrees hotter than that of the water. so the temperature should be 85-90F. Both lights should be on 10 - 12 hours everyday. I have mine set on a timer.
Food:
Your turtles diet should consist of a balanced meal of meat, greens, and commercial foods. But try to feed it mostly commercial foods such as REPTOMIN pellets from ZooMed and greens, not so much meat. And don't feed it iceburgh lettuce or spinach as it is bad for them.
Substrate:
The best substrate in my opinion is no substrate at all. Substrate can be very harmful to a turtle. I myself have my tank bear bottomed. But if you really want gravel then you should have large pieces quite bigger than the turtles head so that he can not eat it, the best rocks to use are large river rocks in my opinion. And NEVER use sand!! Sand is only used in enclosures for Soft-Shelled turtles because they dig themselves into the sand to hunt. But sand should never be used in any other turtle's tank. Sand and small gravel can cause impaction if swallowed and this can cause the turtle to die.
If you have any more questions, please feel free to ask! I love answering them! Also you should join this forum: www.turtletimes.com/forums . That site helped me tremendously when I was researching on turtles. I researched for a year before buying my RES! And this site answered every single question I asked them. And everyone there is very nice. I have been a member since June 2005 there and have posted over 2,600 posts!! I really recommend it