Hey Horapico.kate,
First welcome to the club, You will Love these guys, I think the other one is a painted turtle, even I am not sure as the three kutties (turtles) i have are RES, alright to answer your Question
So the thumb rule is, If your turtle is 2 inch longer (Shell) you need 20 gallons of water. Assuming both of them are 3 to 4 inches you will be needing 30 to 40 gallons of water.
You are right the pet shop owner does not know about turtles, ( Tell him not to sell them :p ) You will be needing UVB light bulbs which you will be getting at any pet store or you can order it online, You also will be needing a basking area for the RES, WHich means apart from the water for the turtle to come and rest, check the other link where it says basking area
viewtopic.php?f=6&t=15260 ,
You can have the light on top of it, If you want the RES to be healthy you can take the turtle to the sunlight for 2 hrs everyday

Hope this helps and all the above is related to the Red Eared Slider, i do not have idea about the other turtle.