Take a look at the Freshman profile for 2015 to see want Stats you need to get into SLO. Since you are a HS Junior expect to see these numbers increase by the time you apply.
http://admissions.calpoly.edu/prospective/profile.html
http://architecture.calpoly.edu/prospective
Cal Poly SLO also uses 9-11th grades in their GPA calculation (different from the rest of the CSU’s) and a MCA point system to determine admission. If you want more information regarding the MCA point system, send me a PM.
I agree that right now your GPA will hinder your chances at the majority of the UC’s, SLO and CPP. Work on getting those grades up and prep for the SAT/ACT. Do EC’s that you enjoy, not just to pad your application.
GPA/Test scores and HS course rigor are probably the most important items for SLO. EC’s will come into play if they are directly related to your major and Essays for the UC’s will be important.
Good Luck.