Peloton has a guide to adjust the bike for your body. However, Peloton instructor Christine D'Ercole has a great video with more direction that I recommend.
I didn't take this and really wish I did. It helps you understand the bike and experience a variety of classes.
You will be FAR more comfortable if you don't wear underwear with your bike shorts or leggings. Below I link to my favorite size-inclusive leggings and shorts for the Peloton bike.
For comfort, sit further back than what feels normal. I am 5'3" & size 14 with a large rear. Half my rear hangs off & almost 3" of seat shows in front.
Don't assume your friend's favorite instructor or style of class will be yours. For example, I found I like endurance rides over shorter rides, riding after work instead of the morning.