You will enter the train station you're departing from to find a huge open area and giant billboards with the departure information (destination, train #, departure time, quai the train is leaving from, etc.). These are constantly changing and it's a bit fun to watch as the signs aren't digital but use the old school style like your old radios (not sure what it's called).
You can get your ticket from an automated machine or from the ticket counter (you can pre-purchase a ticket online). Tickets are by class but do not have reserved seats (except some tgv fares if I remember correctly).
When you enter you go to your car (if you have one, or any car with seats for your class - which it generally says on the outside by the door). If you're running late you can just hop on any car and then just walk through them until you get to one with seats for your class.
There is no limit to luggage and no one actually goes through it or does anything with it... you store it yourself and have to watch it yourself. There is overhead storage and some in front/back of each car (although I remember this being blocked off after the Madrid bombings). You can fit pretty large luggage so pretty much whateveryou can carry you can take. Also, you should be able to find carts which you can borrow for 1 euro (or something like that).
Toilets are generally every other cart and if it's a long train they probably have a food cart. You can find seats that are benches facing one another, bucket seats, or get family rooms that seat 8-10.
Enjoy your trip and bring some wine and cheese to enjoy with friends and it will go a lot faster!