I always use the Voyages-sncf website and nowadays the print at home tickets are ideal for visitors to France. It has its detractors (and its shortcomings) but it is really the only place for the cheapest tickets (prems, IdTGV etc). Once you've used it once or twice you will get to know what all the language means - as has been said - it is pretty self explanatory.
As with all ticket purchase, always strike as early as you can to get the very cheapest.
Having said all the above, I just did a ghost ticket ‘purchase’ on tgv-europe.com
http://www.tgv-europe.com/en/
You can tell from its layout that it is the same as the French site but with English language, rather than being Rail Europe’s UK orientated version (which in my experience gives worse deals, which is why I never use the English links diverting me to Rail Europe
Avoid the temptation to be diverted to Rail Europe (a pop up box gives you the choice)
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