HELMET
Helmets are absolutely the most important piece of gear to wear. If I can only convince you to wear one piece of safety equipment, it would be a helmet. One hit to the skull can end your life. Never ride any distance on a motorcycle without head protection. When I crashed, my head hit the ground pretty hard. I may not have lived if I hadn't been wearing a helmet. Thankfully, since I was wearing a helmet, my head was completely fine.
JACKET
I alway say "Concrete hurts at any speed." Having a thick, tough jacket is important for keeping your upper body in tact. If you go sliding across the ground, a shirt or hoodie will be shredded in an instant leaving only your flesh to be sanded against the cement. Leather jackets are very good for this purpose, but they can get very expensive. There are armored jackets and other options as well that can be a little cheaper.
GLOVES
Gloves may seem like a small thing but are actually very important. In the event of a crash your instinct may be to catch yourself on the ground and without gloves your hands will be shredded. If you have gloves with knuckle protection they can even save you from breaking your hands. Additionally, when on your bike there is constantly bugs and debris hitting your hands that hurt at high speeds if you're not wearing gloves.
PANTS
Tough pants are important for many of the same reasons that a jacket is. Sweat pants or softer materials will be shredded on the concrete. Shorts are even worse as they give no protection at all, leaving your legs completely exposed. Jeans offer solid proection not only in the event of a crash, but also from hazards on your ride. The hot metal parts on your bike can severely burn your legs without thick pants to cover your legs.
BOOTS
Boots are very important since your feet will touch the concrete at some point during your ride for sure. Having slip resistant boots can help with surfaces covered in oil and other slippery substances. Additionally, if your foot pegs are low then it may not be uncommon for your feet to scrape against the concrete when turning tight. Tough boots keep your feet from shredding. In a crash, sturdy boots may prevent an ankle injury.