Start with a practice test, just to know where you are right now. You can download GMATPrep for free and that gives you two tests. Take one and save one for closer to your real test date. (And, if you want to take more, research what's available from the various test prep companies and buy more tests for the in-between period.)

GMATPrep is the best in the sense that it's got real past test questions and is made by the people who make the real test. It's not so great in that it doesn't do any analysis for you - you need to go do that yourself. So you're going to have to go back through and do a tally to see what types of questions you got right vs. wrong to get a sense of your overall strengths and weaknesses. (Incidentally, if you decide you need more practice tests, make sure that whichever ones you purchase provide detailed assessments / analyses of your tests - you need that data to determine what and how to study!)

From there, you can devise your study plan based on your own strengths and weaknesses, but I would say to mix up math and verbal a little bit. You don't want to spend a month doing nothing but math and then a month doing nothing but verbal. More like: a day on math, then the next day on verbal (or two on, two off).

Also, I applaud you for being practical and just focusing on what you need to do to get the score that you need. A lot of people think they have to get some astronomical score and totally stress themselves out - when it's not even necessary for their particular situation!