Syd, welcome to the forum, glad you found us. Everyone here is very friendly and always willing to help out.
If your wanting to do bike seats, you are probably going to want to use 5/7 oz Veg Tan leather. This will give you a good thickness for tooling and carving, but not too much that the seat will be hard as rock. Some people use a heavier leather, and you'll most likely get a few different answers. The best thing is, you can try them all, and see what works best for you.
Have fun, and remember to take pictures.
Ken