It depends on what the purpose of the recording is. If it is so that you can hear your playing -- and there is no better way to find all the problems -- then the microphone that is built into your computer, if it exists, or any one of the really cheap add-on microphones will do just fine. So, for that matter, will the recording software which comes with most computers.
However, if the purpose is so that other can hear and appreciate your playing and your instrument, then you will need to have a really good studio microphone or two, and stands for them; piano is one of the hardest instruments of all to record. Further, your built in software just won't do; while it can make so so .mp3 files, the compression used is pretty poor with regards to fidelity. Better software is available. Neither the microphones nor the software is exactly inexpensive, however.
So -- it really depends on what you want to do with the recording.