I took the Zertifikat Deutsch two years ago at the Goethe Institut Berlin. It was pretty tough for me since I was just starting out, but did manage to pass with a good grade with some work.
Since Takkat already provided some resources for you, let me give you some advice which is specifically tailored to the Zertifikat Deutsch.
The following advice is relevant so long as the exam remains bound to its 2009 rules (it is at the moment, but since this answer will live forever, people should check before taking what I write below too literally).
Do take into consideration what Takkat has written. However, and I can not recommend this strongly enough, be very very harsh on yourself when correcting.
As to your question about whether 'the idea' is enough or to obsess with the grammar: I can not emphasise the latter strongly enough. Just 'an idea' or even 'essentially correct' is not good enough. An interesting, evocative sentence with a grammatical error is worse than a boring plain sentence with no grammatical error. I would recommend that, even if you have a large vocabulary, you only write sentences in the exam which you are 100% sure are completely grammatically correct. This means every declination, every gender, everything.
The listening component is difficult, not because of the large vocabulary, convoluted situations, or strange grammar used. It is difficult because people are talking quickly and in 'every day' situations. The only way to study for this is to be exposed to German spoken in a real-world situation. This means not spoken to you if you are a learner. I would recommend watching TV, listening to the radio, and watching movies without subtitles.
The letter writing component was heavily weighted in my test. Lots of people who study a lot choose to write long letters with a lot of detail and manage (at best) only a couple of grammatical errors. This is the wrong decision. You want to write a few practice letters in standard situations (coming to visit, organising a party with a friend, writing to an old colleague, etc) which are less than a page, direct and to the point, with correct sign-on and sign-offs, in the correct tense and completely 100% grammatically correct.
The speaking component is not very difficult in comparison with the other two. You are allowed a few minor grammatical errors while speaking, although some things are heavily penalised. With all your work for the other parts getting the grammar right here should be a cakewalk. You need to not be intimidated by speaking German with native German speakers.
Caveat: At the B1 level you are expected to know certain grammatical constructions. While I recommend only writing sentences which are 100% grammatically correct, you must also give at least a couple of examples of the more complicated constructions when writing the letter and when speaking in the oral exam. Practically, this means learning a few standard sentences with particles like "entweder", "um .. zu", "so .. dass" and so on.
I hope this helps. Good luck!