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The best as far is space is the bigger the better. 10gals to every inch of shell is the minimum but if you can go bigger the turtle will greatly appreciate it.
Live plants and river rocks would be your best option for decoration or even sand.(note*many people like sand it can help with filtration but my Cumberland slider got a partial impaction from eating sand so idk how safe it really is)
Get a good filtration system. A canister filter is the best way to go it will keep your tank exceptionally clean which is good for the turtle to help prevent parasites and other ilnesses and good cause you clean it less often. They can range from 70$-250$ depending on the size you need. Adding a few very fast fish to your tank can make for a more natural feel for your turtle.
I'm sure others here can help you further but this is basics I'm still learning myself.
Also make sure you have proper lighting(both uva and uvb bulbs or an mvb to create both.if you are going to use an mvb u def need a larger tank)