How can i prevent infection of large scrapes on my right side of my body without taking antibiotics? i fell on pavement off of my bike at ~15 mph.

Scrapes : If you just fell and scraped your skin you need to wash the area with soap and water, apply an antiseptic such as bactine spray, apply topical antibiotic such as neosporin then cover the area with sterile bandages and see your doctor.

Local hygiene: Every day you need to wash the areas with soap and water and then apply some over the counter antibiotic ointment to the areas. Then cover them with some type of a bandage. Don't leave them opened to air and allow them to dry out. You should also check on your tetanus status and obtain a tetanus shot if it has been longer than 10 yrs since your last shot.