This is a very brief tutorial... here's some working code that I used in an altimeter, to grab pressure and temperature from the Sema barometer chip and convert it to altitude (assuming 1013mB sea level pressure. You'll need to do your own conversion from that to height above ground or flight level...)
The chip is smart; it has a high precision pressure cell and temperature sensor, includes an ADC for both, and uses a complicated algorithm to apply factory set correction parameters (different for each example of the chip) to provide the correct output.
Rather than trying to explain it all, I'll attach the PDFs from Intersema, and the working code. One thing to be aware of: the device is extremely sensitive and as a result the output can be very noisy. If you're looking for a fast high precision output, you're going to need some very good filtering. I've used it on an altimeter that I flew with, filtering over eight samples, and it's been ok that way.