The TFT LCD screen you bought is great for experimenting but it "raw", i.e. it is missing the hardware interface required for converting HDMI digital video into its own data format.
The Raspberry Pi supplies a digital video output only in the form of HDMI; RCA connectors are analogue. The HDMI bus can not be connected directly into a raw LCD screen without going through an electronic driver circuit.
Building such circuit is not trivial. In fact if you are a beginner in digital electronics, it could become quite complicated if the timings and/or voltages are different.
I don't have access to the datasheet of your display and the link you supplied does not show any datasheet info. Here is the HDMI pinout from Wikipedia: http://en.wikipedia.org/wiki/HDMI and here is an example of a (random) TFT-LCD connector pinout: http://pinoutdiagram.com/tft-lcd-module-connector-pinout/ There is no guarantee that your panel has a similar pinout!
So to get started, read the signals that your own TFT-LCD panel requires and then read the signals that the HDMI connector supplies. Unfortunately, if they are too different then you would have to search for a different LCD panel or you risk destroying both devices permanently. But if they are similar then it might be quite easy to interface the two by matching the signals. So if you are mentally prepared to accept the risk, you can have fun trying it out.
I will repeat the warning again: If you get the signals wrong, you risk destroying both devices permanently.