MGT1 Psionics gives you a couple different options.
Normally, you roll it 2D6 like any other stat.
However, one important consideration not really mentioned in the main books, but discussed quite a bit in Psionics is: How common is psionics? strictly reading the main books, you get the impression that EVERYONE has Psi. In the Psionics book, there is discussion of how common you can make PSI ability within your setting.
LOTS of Psionic careers there too.
If you are really interested in adding PSI to your game, I strongly recommend the Psionics book. You may not want to all all of it, but it is a great toolbox.