What site would you recommend using to book Trains in France?

www.sncf-connect.com, SNCF is the company that actually runs the trains. Although for regional trains you might just buy the tickets at the station.

Yes, We are travelling from Nice to Paris and I'll definitely buy that in advance, but our trips from Paris to Giverny and other spots just outside of Paris I may wait, if I can confirm the prices don't fluctuate. I just want to plan my day, mostly, by see the correct timetables and prices.

SNCF Connect is a pretty good app afaik, has worked well for me on several trips to France in the past couple years. I like that it's the most direct as well, not a third party reseller.

For SNCF Connect, I’d suggest downloading the app. I had some problems with the website, but the app worked well for buying tickets. One of the nice things with the app is that you can get alerts about train delays real-time. On several occasions, I bought tickets using the app when I arrived at the station.

Great advice about getting the SNCF App to buy your train tickets. The App seems to work more smoothly and it speeds up the online ticket-buying process. www.SNCF-connect.com is where you’ll get the best deals for the TGV trains throughout France by buying your ticket 120 days ahead of your travel date. This is when the “Prem’s “ tickets first go on sale online at discounts of 70% to 80% off the regular TGV fares. This quickly can save you hundreds of euros even if you’re only going to a few destinations in France.
For daytrips— yes— you can simply buy tickets on local and regional trains the day of travel. To go to Giverny, buses are 40 minutes faster and less than half the price of trains going there. Be sure to check a route map of Paris’ RER commuter trains which not only travel from CDG Airport to central Paris but also go to Versailles.

I only book using SNCF. Hell, they're the ones that actually run the trains. But keep in mind that if you try to book too far in the future, you won't get an accurate idea of what trains and schedules are available. Waiting until three months out is usually safe.

Thank you everyone! It's so much easier to use an app or website than to do it at the train station. You know if they're seats or not.

We used trainline.com this week to go fro Paris to Strasbourg. No issues.

Trainline.eu is easier to use and gives a more user friendly look at options and prices. We have found it sometimes a bit cheaper although that makes no sense. I got frustrated with the SNCF site when a train I booked turned out to be a bus which was not clear until after booking. I cancelled that one, went to Trainline and was able to book a train. One strategy is to use the Trainline site to figure out options and then book on SNCF.

Jane is onto something, research with Trainline or Rome2rio then book elsewhere.

Or walk into a train station the day before.

Based on my experience, Rome2Rio cannot be trusted to provide accurate information on schedules, fares or trip frequency. I wouldn't use it for anything except possibly finding the name of the bus company serving a particular route--and even there, Rome2Rio may not have complete information. In fairness, bus info is harder to find than train info.

Just purchase the tickets through the SNCF website.

For the high speed trains between Nice and Paris, the tickets will go on sale 4 months prior to your departure date.

Trainline.eu used to be "Capitaine Train" before it was bought out by Trainline UK. It was setup by a bunch of French guys who were net very satisfied with the SNCF booking process. (eg. SNCF at one time could not book you on a trip that involved more than 2 trains). It is still quite popular in France.
Also a good choice if you want a ticket from somewhere in France to eg. Italy, Germany or Switzerland.

With trainline I was always able to get a PDF of my ticket and save it to my notes wallet -- with SNCF if was frequently difficult to get a downloadable ticket that worked for me. I am sure others do better with this, but I found it endlessly frustrating which is why I usual book with trainline.