Cyrille,
As Sherwood suggests, start with the EZ stuff.
That would be:
1. Pull up the rubber boot from your shift lever and check to see if the tabs are bent (just like emcdan suggests)
2. Adjust the coupler see here how that's done
http://www.pelicanparts.com/techarticles/911_shifting_improvements/911_shifting_improvements.htm
That would take hardly any time and a toolset of one screwdriver to open the access panel between the seats and two 13mm wrenches to losen the clamp bolt from the coupler.
If that doesn't do it, prepare to change out the bushings... not expensive and very easy.
Good luck,
Michiel