I'd at least go through the OG12 first before your "burn" your first practice test. Give yourself at least 2 weeks to build up your basic content knowledge before you take PT#1. Any practice test is fine -- Kaplan, Grockit, MGMAT, etc.
These are my basic "getting started" tips:
1. Start with the official materials. How will you study? The most successful GMAT test-takers use a variety of materials. Bare minimum, you'll need the Official Guide, the downloaded Powerprep CATs from mba.com, 1-2 other workbooks from reputable companies like MGMAT, Barron's or Kaplan, and several supplemental CATs (also available from private companies - Amazon is a great place to read reviews).
2. Take advantage of online resources. If you want to go the extra mile (and are aiming for 700+), use online resources like Grockit, Beat the GMAT, the "Sample GMAT Questions" from mba.com, and other online forums to seek out a motivated study community.
3. Schedule your study time methodically. Buy a white-erase wall calendar and write your test date in big letters at the top, then begin to schedule study time for yourself by working backwards. Aim to study at least 1 hour a day, 5-6 days a week. Don't try to cram 20 hours of studying in one week and then do nothing the next. Slow and steady wins the race. Focus, but don't burn out.
4. Register early. Choose a GMAT test date that gives you a comfortable window in case you want to re-take. It's important to set your test date first. If you wait until you feel "ready" to take it, you never will.
5. Build content knowledge first. Don't try to do everything at once. First familiarize yourself with the tested subject-matter. The OG is the best place to start. You may want to buy supplementary algebra, geometry or English grammar books to fill gaps in your knowledge.
6. Work on creating a strategy for each question type. As you begin to feel more confident with your foundation in content, practice different strategies for each question type. Read articles online at Grockit's blog for suggestions. You may need to try several methods to see what works best for you - make sure you are using your scratch pad, especially for CR and RC. Write down your strategy for each question type as a numbered process. If you can't do this, chances are you don't have a strategy in place.
7. Use a tutor your weak areas if needed. Consider working with a tutor for a couple hours to hone your technique. Strategy is the most important step in your preparation because it dictates how you are actually taking the test, so if you are struggling with a certain area or question type, you're not alone! Reach out to an expert for supplemental help.
8. Work on pacing. Don't worry about your pacing until you've reviewed the content and you have excellent strategies with high accuracy. If you try to move too quickly too soon you will likely never move beyond a limited accuracy. Once your strategies are good, try to set an "ideal" time for yourself and practice with different sets of questions.
9. Take full-length tests. You'll want to take at least 6 over the course of your studies - save the two official Powerprep CATs for the last quarter of your study-time. Use a private company's CATs to practice your pacing and for extra question review but do NOT put much faith in the "scores" they give you. No private company will accurately mimic the official GMAT's scoring algorithm, and too often students are disappointed when they score much lower on the actual GMAT. Don't let these CATs lull you into a false sense of panic or security. Use them only to build your stamina.
10. Don't neglect AWA. Write both practice essays each time you take a practice CAT. Send them to friends to review, post them on an online forum, or schedule a session with a tutor online, or an in-person tutor, to review them and ask for suggestions. Make sure you are developing a template for both essays that can easily be completed in the allotted time.
