Some will apply rub and put meat in the cooker immediately. Others will apply rub and refrigerate overnight. Others choose an interval in between. I like overnight, but I would suggest at least 2 hours.
Rub for a brisket can be as simple as kosher salt and freshly cracked black pepper. Use your imagination, a good recipe (you'll find several in the Recipes Forum), or your favorite commercial rub beyond that.
Regards,
Chris