how about starting with grammar basics. learn one concept a day for the first few days, then increase it to two concepts a day.
your first step should be to analyze your mistakes. See where you go wrong. Are most of your mistakes in Subject-verb agreement? Modifiers? Or is it that you did not do a lot of work in Grammar before taking the test.. these are only some of the questions that you need to explain to yourself, before you move on. The idea is first find out whats wrong and then think about fixing it.
If you do not have much time, I strongly recommend you to consider GMAX , the online prep course for GMAT.