Is it best to make an advanced reservation online or just to wait and buy tickets there?
Advanced registrations will be cheaper, but they usually limit you to taking a specific train. It depends if you want to plan that rigidly.
The standard 2nd class fare for a direct TGV from Paris to Reims is €32.50. The cheapest Prem's fare is €12.00. Those tickets are non-exchangeable and non-refundable. Book NOW at www.tgv-europe.com. To keep the site in English and to avoid being bumped to the Rail Europe site which usually doesn't offer discount fares, choose Great Britain as your country of residence. Print your own tickets.