Actually, I wouldn't do any diagnostic test or practice questions yet. You'll waste a test if you are completely unfamiliar with the GMAT.
Read through the PR book to get an overview of the test and the types of questions and traps you'll see, then take your first diagnostic to see what your strengths and weaknesses are. Then start doing practice questions, one type at a time each night for 2-3 hours. After you've done that for each question type, take your next full test.
Review that test carefully to further hone in on your weaknesses (they may be general test problems, or specific to a question type). Then continue with practice questions. I would do practice questions during the week and one full test each weekend. If you haven't downloaded the GMATPrep tests
, definitely do that -- they're free from http://www.mba.com
As you go through the OG, the questions get progressively harder, so go straight through. If you need additional practice questions, there are some great ones here in the Challenges forum. You can also get separate verbal and math workbooks (I have PR's for both and would recommend them).
There is a sticky post on the home page of this GMAT forum about how to do an error log
. Peruse that...very helpful.