yes you do
there will be no rails on the bottom of the seat.
It's how the saddle attaches to the seat post. Regular saddles have two saddle rails that can be slid fore-aft in the clamp Pivotal saddles are kinda bolted directly to the top of the post instead. Makes for a stronger connection but with less adjustability.
I would suggest a bike with a small frame and a long seat post if you want a low seat for jumping, but a larger frame if you ride mountain and street more.
Usually simply by opening the seat post clamp and sliding the seat post down. Sometimes by either cutting the post down to make it shorter or by buying a shorter post.
I can't even guess what you're asking about. You need a seat post of the right diameter for the frame, the right length for your needs, and preferably one suited for your type of riding. Using a road post on a MTB is not advised. If you want to get picky you can tinker around with set back and whatnot.
usually the size is located on the tube where the seat post goes in
Around 28-31 mm, which is rather pointless information to have, as the seat post has to have the right size down to the tenth of a mm to be useful.
Use a vernier caliper to measure ittry to look up the specification for the bikebring the old one to the store and ask for one of the same size.
Simply change the exponents on the bike for lighter ones, like the wheels, tires, seat post etc. this will cost more.
Disassemble, clean and reassemble the clamp. Tighten harder. If this fails, get another clamp, or another seat post, depending on the design.
On a MTB it's usually a clamp that goes around the seat post. On a road bike it can also be screwed on to a soldered tab on the frame.