Yes, I can and so can any Jewish male over the age of 13 at the appropriate time during the thrice daily prayers in a synagogue.For the full text of the kaddish see the kaddish entry link.
Kaddish is said AFTER the Friday night Shabbat service.
To remember their loved ones who died.
To show their respects for the dead. Especially if it was someone important, which was basically everybody during that point in time.
The Kaddish is a prayer for Moshiach to arrive. Since the dead will be revived when Moshiach arrives, this prayer is said by mourners.
That's a personal decision.
How To Understand The Meaning of Kaddish
How to Say Kaddish
Learn the Mourner's Kaddish Prayer
Learn the Half and Full Kaddish Prayers