I was in the same situation, got a red eared slider at a flea market 14 years ago. The Austin's Turtle page is probably the best comprehensive care sheet out there, and I would give it a read. Red Eared sliders are not a beginners turtle, but they are often sold that way. I had mine in a small plastic container too, I was too young to know better when I got it, but get them out as fast as possible. I have a single male in a 60 gallon tank and honestly it could still be larger. For 2 turtles, without exaggeration, a 125 gallon tank will be needed eventually. For the time being I would suggest getting AT LEAST a 20 gallon long tank, that'll last you a few months. To save on costs, it'd be better to buy everything used off of craigslist. The turtles are pretty tough, but they do get quite large.