no contract plans help

Hey guys so i have a question about no contract phone plans. So i went into Walmart and saw all these different cards for different amounts of prices and such. I'm wondering how the no contract plans work. I already have a phone to use so can i just pick up let's say a t-mobile card for $25 or do i have to buy the one that has the sim card? I've always done a 2 year contract with sprint but i don't wanna be sucked into another 2 year contract so can anyone please help a sister out and explain how these work, thanks!

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