Detailed booking steps can be searched online, so I won't go into details. It's not too much trouble to do it step by step. The website of France Railroad is still very useful, which is in sharp contrast with that of Spain Railroad.
You can buy tickets directly whether you have registered a website or not. Trains in France are mainly divided into TGV (French high-speed rail) and ordinary trains. The online booking methods mainly include ticket collection at the counter, ticket collection at the train station self-service machine, mailing train tickets and printing on A4 paper. The first two are not immediate payments. You need to reserve a bank card to get the ticket. Every station has a yellow self-service terminal, which is powerful and easy to use. The last one is to print it yourself. Not every car has this option. Usually only TGV has it.
I hope the above answers are helpful to you.