Need turtle expert help? please!?

Me and my boyfriend are planning on splitting the cost of getting a turtle and we want to know what the "best way to go" about this process is? what turtle wuld be best? what tank size (i know its like 10 gal per inch of turtle and that you need pebbles big enough for them to not swallow)? what decorations for the tank? would an 80 gallon tank be ok for one turtle or would it be too big? should we get two? if they get bbig enough would it be okay to build a pond just for them? whats their faveorite food? how much do they cost (for the turtle you suggest is best)? is it okay to go ahead and set up a big tankk before we get the turtle? how big do they get? can you take them for walks? how often should u clean the tank? how much does the uv light and heater cost? how big should their dock be? are there any turtle toys? give me any information you can!