when Pi Filter should be used and how to choose its L an C value ?
It's used when a low-pass filter is required. They are often used to filter noise from power supplies and in radio transmitters and receivers. In the latter applications it needs to be designed to match the impedances on the input and output.
I use this software for LC filter design, it's very good. The ARRL Handbook has design tables for the amateur radio bands.
A simulator (I use SPICE) is useful for checking a filter's characteristics.
Pi filters (shunt capacitor + series inductor + shunt capacitor) present very low high-frequency input impedance at each end due to the capacitors.
T filters (two series inductors with a shunt capacitor in the middle -- the dual of a pi filter) present very high high-frequency input impedance at each end due to the inductors.
For 50-ohm source and load, it doesn't matter much, and you can just pay attention to the transfer function between input and output. But in other situations, one may be better than the other because of impedance characteristics.