If you are not sure about riding don't rush out and buy a bike. Ask around my bet would be that several people have a bike that often is sitting idle that you could borrow.
Of course a new bike should be ready to ride, where as the used/loaner bike might need some care. Initial rides for most are short, its better to pick a ride around you area, something about 1 mile. This way if something does happen you are close to home.
Ready to Buy new? Go to a Bicycle dealer, they put riders and bicycles together everyday, and you can rent from many of them for that first ride. They want you to enjoy the ride so a reputable dealer will fit you to the right size bike and type. Do a little research, roads bikes are fast and fun, they don't really like dirt and rocks, that is where the mountain bikes shine, then there are cross over models that are in the middle to some degree
Seat time is just that, and it will set the length of ride more than most other components. It will take some time to find the proper seat angle, and position. Height is easy since you want a slight bend at the knee when the pedal is all the way at the bottom. Make changes to see what they affect, over time I have established reference numbers that get a new bike close for the first ride. The more you ride the better you can analyze what is needed to make the ride enjoyable.
Gears, oh as many as 30 on the current road bikes and 27 for the mountain bikes. So what gears to use when first starting out? Use as many as you feel comfortable with, learn how to shift. Knowing to let up while still pedaling as you shift helps a great deal. Great strides have been made in this area over the last few years and it just keeps improving.
Having a steady pedal speed when done correctly allows you to control heart rate and all the goes with it. ,
As you ride more the Cadence becomes valuable, its the RPM and much has been written about it. I ride 80 to 95, its easy to measure by riding for 1 minute at a steady pedal speed counting each time say your right pedal hits the bottom.