Prep Advice Needed

Dear Diego,
I'm happy to respond. This is Mike McGarry, the GMAT expert at Magoosh and author of most of those Magoosh flashcards. I'll say a few things.

First of all, here's a blog you may find helpful as a start:
https://magoosh.com/gmat/2013/how-to-stu ... gmat-math/

Here are a couple of three month study guides that you may find helpful:
https://magoosh.com/gmat/2012/3-month-gm ... beginners/
https://magoosh.com/gmat/2012/3-month-gm ... h-focused/

The math cards are good reminders if the knowledge is somewhat fresh, but if you really are starting from Square One in math, then I strongly recommend enrolling as a Magoosh customer. This would give you access to the entire library of math video lessons, in which I explain everything from A to Z about GMAT math. Here's one lesson for free:
https://gmat.magoosh.com/lessons/1248-multiples
Magoosh has 1000+ GMAT practice questions, and each one has its own video explanation. A great deal of the "teaching" we do at Magoosh happens in those VE. It's one thing to have abstract lessons, but in the VEs, we can be very specific: "in this question, this was true, and not that, so we did this instead of that." That immediate and specific feedback is exactly what someone in your position needs to accelerate. Here's a practice question:
https://gmat.magoosh.com/questions/2613
When you submit your answer, the VE for that question will be on the next page.

Even if you decide not to enroll in Magoosh, I hope you find the study plan and blog helpful Explore that free blog: there's a lot that that can boost your math performance as well.

Best of luck, my friend!
Mike