I am planning to buy train tickets soon for train travel in late September. Is www.thetrainline the best/easiest way to buy tickets?

I hope that after i buy the tickets, the tickets will set for all info I need, such as car number and seagt number, as well as any code I need to show the conductor.

I've used the Trainline website for a couple of trips. They add a small fee to the ticket cost but I've found the site user friendly. Everything you'll ever want to know about train travel, including tickets, seats etc is on Seat61 website.

Well..................
If Tom IS young, perhaps he's never boarded a train.
If so, if I were him, I'd google for a video of someone boarding a TGV in a Paris station, such as Gare D'Lyon, or one or the others. Just to gain some familiarity, to feel more comfortable doing it.

No, it's not the best, the best is always the actual website of the train company in every country. I only use SNCF, the French railway. Trainline may be easier for some, I assume there is some reason people use it. I've never found SNCF difficult but I only use it in French. You have to pick a seat number, but you can't on some trains.

SNCF Connect : Book your train tickets to France and Europe (sncf-connect.com)