First step. Empty out any pre concieved notion of Parkour. Big Jumps and crazy flips are down the road so don't even worry about them yet.
Step two- Basic Conditioning. Leg workouts seem to be the best thing due to the need for explosive jumps. Squats-Dead lifts- Pistols are great. Running for Cardio and speed. Upper body work, Push ups are okay to start, Pull ups. Check the General Fitness Board and the Women Board for lots of helpful stuff and maybe start a training log. People here are very helpful
Step 3-Mobility, spacial awareness and balance are also super important. Look up or create a dynamic stretching program and warm up, this will help keep you safe from injury and is usually a good workout. QM (Quadrapedal Movement), Rolls, Rail walk (or the thinnest AND LOWEST thing you can find). Practice precission jumps by drawing a line on the ground and landing on it. Start touching everything, sounds weird I know, but this improves awareness of your own space.
The fact that you're a girl isn't that important to the true reason of parkour- To be better today then you were yesterday. To grow.
And most important, be safe and Have fun