Okay, I'm going to give you some basic rules of thumbs, then send you off to a turtle forum!
Aquatic turtles - ten gallons of water per inch of shell. That adds up to a LOT very quickly. What species of turtles are these? Mud/musk turtles need very different habitats than sliders and cooters.
Filtration - Get something at least TWICE as big for the tank. As in, for a 20 gallon tank, you'd need a 40 gallon filter. Because turtles are NASTY.
Don't use gravel in the turtle tanks. At best, use rocks larger than their head. Some turtles with eat their gravel and become impacted.
Turtles need UVB lighting AND heat lighting. And a place to remove themselves completely from the water and bask under these lights. The UVB has to be within 12 inches of them to be effective.
The box turtle -
Don't use aquariums for these. They don't quite understand the glass and it's stressful for them. Get a huge rubbermaid tub of some sort. Heck, you can use one for the aquatics too if you set it up right.
Boxes like to burrow. They make something called Bed-a-beast. Its very good. I also recommend cypress bedding. You can buy HUGE sacks of it in the gardening section of
walmart? Calling cypress mulch or something like it.
Boxes need a varied diet of meat, fruits, veggies, insects, etc. They ALSO need UVB and lighting, as well as something they can get into and soak, a pool of some sort, and a hiding place.
These are just the basics.
Here is a GOOD forum:
http://www.turtleforum.com/forum/upload/index.php?act=idx
CareSheets:
http://www.allturtles.com/
http://www.wnyherp.org/care-sheets/turtles/
If you post pictures of the turtles, their shells (top and bottom), heads, etc, I can identify them for you.
Good luck! Raising and keeping turtles is an adventure, and I have to admit, I love turtles. They are my favorites of all the species I've kept.