I suggest that you start with Data Sufficiency (DS). This question type is totally unique to the GMAT, so it's foreign territory. As such, everyone struggles with DS questions at first.

If you're interested, we have a free set of videos that cover everything you need to know about DS: https://www.gmatprepnow.com/module/gmat-data-sufficiency

Cheers,
Brent

The kind of content mastery that you'll need (geometry rules, quadratic rules, etc) will be equally applicable to both PS and DS, so there's no need to study for one or the other "first." Both question types test the exact same mathematical rules, just in different formats.

I think Brent is right, though. You want to understand the ideas and methodology behind DS before you start studying other content, because it's such a unique question type. Learn how to rephrase the question and how to analyze question types.

Then, start learning quant topic-by-topic. For example, using whatever study materials you have, read all about exponent rules and how they're tested. Then, do a set of OG problems dealing with exponents - and alternate between both PS and DS.

I should also mention that I'm a big believer in the "fermentation" process that occurs when you learn something new. Once you've learned a basic concept, your subconscious mind plays around with that concept so that you inevitably come to have a deeper/richer understanding of it.

For this reason, I suggest that you watch all of our free Data Sufficiency videos, and then begin learning the specific GMAT content (geometry, statistics, etc.).

Cheers,
Brent