What are your dates of travel?

You can buy reservations at b-europe, or better at raileurope.com (select ‘add railpass’ to see the correct prices and availability.

You can buy just before travel, as long as there’s availability. 

Oh, great!
I want to travel from January 1st to January 5th
So, to summarize, I need to buy a country pass and then book seats in raileurope.com beetween each station?

If you want to use a pass, then yes you’ll need a pass valid in France. Either a one country pass, if you’re only travelling in France, or a Global Pass if you are travelling in other countries.

You’ll need to do your own research about whether a pass is the best option for you.

Here is a good place to read about them: https://www.seat61.com/how-to-use-a-eurail-pass.htm