Hi rnix,
I don’t know which store you have your credit but the first place I would start is to make sure you read the tutorial post here which has all the basic information, steps, and guidelines that can help you make the best possible choice … and know how and why to avoid the worst ones.
If you have your credit at a typical chain store then you are in a fairly difficult position because none of their mattresses may be particularly good quality or value. If that’s the case then post #2 here may be helpful when you need to exchange your mattress and your options are limited to what they sell.
Phoenix