First determine when you want your test date. Your whole approach will depend on how much time you have to prepare.
Then, I would consider trying Beat the GMAT's free study guides: http://www.beatthegmat.com/mba/gmat-guide
They have various guides based on how much time you have until test day.
Within those study guides they detail all of the material needed. I went with Manhattan's comprehensive package found here: https://www.amazon.com/Manhattan-Complete-Strategy-Guide-Guides/dp/1935707701
Only $35 to rent them all. Additionally for more practice problems I would see if your local library has any official guides.
Between Mahattan prep, the official guide, and Beat the GMAT's designed study guides, you should have enough resources to perform well.
If you're still struggling with content after doing self-guided training, only then would I consider taking a class or finding a tutor.