Start small and low impact, with balancing, quadrupedal movement, jumping, and safety vaulting. These four very simple movements, paired with basic strength & conditioning exercises (body weight exercises), is a great point of departure. You’ll be building critical skills, such as balance, control, and body awareness, which will translate later into all of the more complex movements.
I think its really important to just stay open minded. Literally anything can be utilized for Parkour, especially trees. I grew up in the suburbs and attended college in the tundra, so I learned to make the most of both sparce & indoor conditions. Benches, couches, chalk, curbs, all can be utilized. I even built a vault box for my yard as well as a set of precision trainers & rails. You can learn more about how to build your own equipment online—Sturdymade is a great resource, and there is also a great facebook group as well where people who build PK equipment talk together about how to build better!