You will have much better success with this project if you clear out or at least cut back to the ground as much of the existing vegetation as you can. Seeds must come into direct contact with the soil for proper germination.

A lot of native areas are maintained by regular burning. If that's not possible, mowing that area before seeding will reduce or slow down the competition from taller plants. A lot of people also frost seed and claim they have better results. That's when you wait until there is snow on the ground, and then you just seed into it. Like gardengal said it's best to have the soil open for good seed contact. You could clear out some small areas with particularly bad weeds or invasives. You want to get rid of any brush or trees that are trying to take hold. They will continue to advance and suffocate any desired plantings. Maybe do an assessment of that area to see which plants you are dealing with.

Unless city practices poisoning of everything, or high storm water flow reliably will occur (run seeds away), then go ahead and prep.