I have to create simple device for a promotion campaign. I should react when someone seats on the chair, so I need advice on choosing the right sensor. Should it be pressure sensor and what kind is better for Raspberry Pi?
If you just need to detect someone sitting on a chair, you just need to connect a pressure-switch or mat to one of the GPIO pins & read the status. There is plenty of info on this on the RaspberryPi forums.
A robustly-sprung push-button switch on one of the legs could also do.
A microcontroller running a large capacitive sensor under the seat cushion sounds like it would work. That can then communicate with another computer in various ways. Via a serial port is probably the easiest considering both sides. If your computer doesn't have a serial port, then use a off the shelf USB to serial adapter.