1. Identify what object is causing the friction on your system
  2. Determine if your system is moving relative to the surface. If it is, use kinetic friction. If not, its static friction city.
  3. If it's kinetic friction, the direction of the friction is opposite the relative motion. If you have static friction imagine the problem without friction and ask which way your system would move relative to the object creating the friction.
  4. If you are using kinetic friction, the magnitude is \mu_k times F^N. If you have static, then determine if you are the problem requires the maximum static friction, which is \mu_s times F^N, or if the magnitude of static friction is less than the maximum.