Those are all great things to do! We have students in our classes take a test right at the beginning, too—as you said, you want to see what it's like, and you also want to get a sense of your natural strengths and weaknesses.
Here's what I'd recommend, generally speaking. Look over your CAT. You can re-do some of the questions, and mark the ones that you now know how to get right (from doing Foundations of Math, etc). Categorize everything into one of three buckets:
(1) These are my strengths (relative to everything else)
(2) These are weaker areas but I feel more comfortable trying to improve them (include careless mistakes or "I should have known that!" stuff here)
(3) These are weaker areas and I don't feel all that comfortable trying to improve them
Start with (2), mostly, while also pushing (1) whenever those areas are covered. Leave the (3) stuff for a little later (or possibly never, for some of it!).
For quant, consider starting with stuff from FDPs (Fractions Decimals & Percents) and Algebra, as this material can also be tested in the other content areas.
For verbal, I'd generally go through SC in the order of the book. For CR and RC, generally start with the introductory chapters first, but when you get to the question types, you can jump around if you like (just keep the material in one chapter together).
The OG = The Official Guide. These are books that are published by the makers of the real test, so if you only bought MPrep books, then yes, these are different books that you would have to buy. I strongly recommend using official questions when practicing; Official Guide questions were all used on the real GMAT in the past, and there's nothing better than studying from the real thing. You can get just the one main book (with about 900 questions in it): The Official Guide for GMAT Review. (There are two others, with 300 questions each.)
You can also download two free official GMAT practice tests and some other free resources from
www.mba.com (the makers of the GMAT). Look for the GMATPrep software, which you can "buy" in their store for $0.
Okay, next, what to do. First, work through a chapter in one of our strategy guides. Then try some of the questions in the end-of-chapter problem sets. Consider these questions "open book": you can go back into the chapter and look anything up at any time while solving these problems.
If you decide to buy the Official Guide, then you can also download a PDF on our (MPrep's) website called The Official Guide Problem Sets. We've categorized all of the OG problems by book and chapter. So, when you finish a particular chapter in one of our books, you can go to the OG Problem Sets PDF and see which question numbers correspond to that material. Then you can try a few of those. This time, hold yourself to official test protocol: time yourself, don't look anything up, etc. When you're done, analyze the problem to see what else you can get out of it:
http://tinyurl.com/2ndlevelofgmatThat will also let you know if you need to go back into the book to review or drill something. As you move deeper into a book (or several books), start to put together small sets of questions and give yourself a whole block of time (eg, you might give yourself 8 minutes to do 4 quant questions). Mix up topics—stuff you just studied, but also stuff you did yesterday and last week. This will help you to review and keep your skills current.
You might also want to take a look at this:
http://www.beatthegmat.com/mba/2015/07/ ... s-say-whatWhen you feel that you have made good progress* and you want to take another CAT, do so and use this to analyze it:
http://tinyurl.com/analyzeyourcatsThen use that analysis to figure out what else still needs work and go from there.
*In our classes, we have students take their second CAT around week 6 or 7 and their third CAT around week 9 or 10, just to give you an idea.
Hopefully, that is enough to get you going. Let us know if you have any questions!