Barb,
DB is the German Railways site it will only sell tickets for journeys within, to or from Germany.
The general rule is you have to book on the site of the Railway company running the train you wish to travel on. But, I agree, the SNCF site is particularly user-hostile. For France (and some journeys in surrounding countries) there is a private site selling tickets which is a lot more user-friendly: https://www.capitainetrain.com
The trick with SNCF TGV tickets is to book 2-3 months in advance, then you get cheaper fares.
In general, if you want to find out how and where to book rail tickets, the best site with all the info is the abovementioned: http://www.seat61.com/ It even explains how to use the SNCF website!
On some routes you even have rival companies running the same route in competition, and you have to book on the right site (rather like airlines).