If you aren't very high in Quant, you should start with Foundations of GMAT Math. Then you can jump into one of our main strategy guides, such as Fractions, Decimals, & Percents. (That's what we assign first in class.) Even if you are strong in quant, I recommend that you use the Foundations book throughout your study as a source of review/warmup drills.
On the verbal side, we also have a Foundations book you can start with. You should also do the first few chapters of the Sentence Correction guide, because you will want to work through that book over time and absorb the rules bit by bit.
For each chapter in the main strategy guides, you will find a list of corresponding Official Guide problems in the back of the book. You can also try the GMAT Navigator database on our website.
That should get you started! Let me know if I can help with anything else.