If you have never gone backpacking before, do you have gear?
Look on Meetup.com - there are a lot of hiking groups that go backpacking in Yosemite, from all over California. I am part of two of them. Last backpacking trip we had someone from Boston join us while she was in town on business. Also check the Tehipite chapter of the Sierra Club - there are backpacking outings on their schedule and you do not have to be a member to go.
As for preparing, for Yosemite wilderness info you can look at the park website,
http://nps.gov/yose, there is a ton of info on regulations - you need to especially follow the food storage, some bears are year round nuisances. In terms of backpacking gear... look at the articles at backpacking.net. I submit that because you are not 20 and indestructible that you avoid traditional gear from REI and similar and consider lightweight backpacking - as a person of good health who came back to backpacking after decades of desk jobs, I can tell you that getting into this late as you are, a 20-30 lb pack will be quite enough, 40-50 will be impossible and lead to aches and pains you didn't know you could have. Avoid 6 pound 85 liter packs that will invite you to load it with camp chair and look instead for 50-60 liter packs of 3 lbs or less... Unless you are renting gear, in which case you will get what they rent you. Beyond that your budget and your intent to continue backpacking or not should be an issue - I backpack all the time, so I invested considerable effort to research and purchase the right things, light and compact enough to fit in a 40 liter pack, resulting in a 25 lb 3-5 day pack.
And elevation kicks your butt. I have had hikers in my groups who hit the gym three times a week gasping and faltering because of a change in elevation. Can't predict who elevation will affect - so if you don't live over 5-6k and you can dayhike somewhere over 7,000 feet before you go, do it.
October can be difficult because the park services shut down to a minimum starting in September. By mid-October you will be unable to park on Tioga Rd, and shuttles are weekends only. You will want clothing and gear good to 20F at night.
I would suggest a loop hike or a simple out and back from the valley or one of the trailheads in the southwest of the park, and for your first backpacking trip... something that is shorter rather than longer. Ideally you would be hiking and doing a low mileage overnight well before you come to the park, to test your gear and yourself in the wilderness first.