If you're just learning a language, start out with something easy! Assimil courses are great, because they have lots of short little stories with translations. You can also try bilingual children's books and anything else with a translation.
Don't forget to choose interesting texts! You'll being seeing them a lot.
Once you reach intermediate level, you should try lots of fun native materials: books, comics, websites, news articles, Wikipedia and transcripts of TV shows.
You don't need to worry about rare words that you won't see again soon! Start by making cards for:
Remember, you only want to see 10 to 20 new cards in Anki every day, and you should be reading a lot! So be selective. But don't worry: If you create too many cards, you can just delete them later. See below for more information on deletion.
These are the easiest kind of card: All you need to do is understand the boldfaced word in context.
What should go on the back of this card? If you're just beginning, you may want to include a complete translation:
As you become more advanced, you may only want a translation of a single word, or even a definition from a native dictionary:
When reviewing these cards, mark them as "Good" if you can understand the word in context without any problems.
This type of card is inspired by two things: Those horrible fill-in-the-blank exercises we all did in school, and by Khatzumoto's series on cloze deletion cards. These cards actually require you to remember something actively, but they're ridiculously easy.
First, we hide half of word on the card, and supply a hint of some kind:
Then we create a second card which hides the other half:
You would think that these cards are too easy, but in practice, you should be able to recall the entire word without any problems. And you shouldn't have any problem remembering which synonym you're supposed to use on a given card, because half the word is already there.
When reviewing these cards, mark them as "Good" if you can fill in the blank. You're not even required to understand the rest of the card!
Whenever you look at a card and say, "Ugh, not that card again," delete it!
If you read enough in your new language, you'll see any important words again soon. And you definitely want to keep Anki fun! If your deck fills up with bad cards, you'll stop doing reps all too soon.
I recommend Khaztumoto's excellent article on deleting cards.