I don't know how to critique
Critiquing is not a question about talent, but practice! If you feel you don't know how to crit, read the instructions HERE.
You just do your best and with time you will be able to give crits that are both invaluable and useful for the author who uses them to improve his text.
Remember to be constructive, the crits are meant to help the author, not tear him down!
It can be very helpful to read other people's crits (see here)