Learn how to bunny hop properly. Recording yourself with your smartphone will make it easier. Once you learned to bunny hop, start jumping soft obstacles (water bottles, cardboards), higher and higher. Make the city your playground and you are doing BMX. Next tricks would be fakie, 180 and barspin in this order. Be careful with the cars in the street corners as you dont have breakes, also you should always do a little warm up before a session to protect your joints health. Helmet is a good choice too, but mostly take care of your joints. Read about articular cartilage wear, meniscus tearing, ligament torn. This is an extreme sport, injuries are very very common, everyday you hear about someone losing a thooth, breaking a bone, having knee injuries, ankle injuries and I have seen close-to-death +2 meters falls without helmet. I had a knee injury myself, so I kept doing BMX for some months but I realised I should look for a less joint-aggresive sport since this kind of injuries will go along with you for the rest of your life and I didnīt want to have a worse injury the next time. Take care of your health, have fun.