There are very few free resources to find English translations of foreign opera libretti (do a google search for "opera libretto" and it will throw up a few decent sites, stanford university is good, but most only in the original language).
There is an excellent series of opera guides, recently republished, which include on the whole excellent translations as well as comprehensive guides to the opera - search on Amazon for "English National Opera Guide".
Most CD recordings include a booklet with the full libretto in the original language and English - if you can't afford to buy the CDs, use your local library - many good recordings can be borrowed for a small charge, and you should get the booklet with the English translation in too! Or rent a DVD of an opera, and follow the subtitles as they sing, even easier! There is also a recent series of recordings on the Chandos label, which are sung in English, and these are on the whole excellent too, so plenty of ways to help understand what is happening!