well firsty it depends on your tank size. the yellow bellied turtle shown on this vid can grow up to 13" if its female. the brownish colour one up to about 7" if famale. 5" if male.
secondly it depends where you are located. there are some laws prohibiting the sale of certian species of turles as mentioned above.
you will need a large tank (10gallons per 1") of turtle. secondly. very heavy filtration. or requwnt water changes.
you will also need a basking spot in which the turtle can get compleatly out of the water to bask under a specific UVA emmiting repltile light (available at most pet stores) and a basking lamp over the basking area.
thebasking area can be as imple as a piece of cork wood that is sectioncupped to the side etc.
i do not use a basking lamp as the basking area is located close enough to the UVA lighting that it emmits adiquate heat. (85 - 90*F)
you may need a heating in your tank, depending on species.
if you are located in the UK or somewhere warmer ( which narrws it down to anywhere on the globe)
turtles like sliders can be kept outdoors all year round but wll need to hibernate.
if you do not what an indoor turtle i suggest a male mississippi map turtle or a male false map turtle. (females grow larger) which will be able to survie in a 40 gallon + tank. a musk turtle is another option which stay small at about 4" for each sex. more information on this site.
turtle page
Turtles are awesome you'll have lots of fun with them I myself have had my turtle for a little over a year now it's a Red Eared Slider (RES) it's very similar to the Yellow Belly Slider (YBS). But I hope you've done your research because turtles arn't the easiest pets to keep. This is a great site to ask questions on your turtles and all that jazz: www.turtletimes.com/forums It's is a lot like this site. My username is the same on that site if you want to talk to on it BTW 
