yeah wait until you atleast take physics and biochem, unless you have already taken those courses. The kaplan practice questions are terrible and hard for no good reason. Just be patient, I have the kaplan books and I found them overall pretty helpful. Nice way to review all the material. Finish the content first, don't waste any practice tests yet bc it will only make you feel demoralized. Finish the content, then take practice tests as many as you can. Id say buy NextStep after you finish your kaplan tests and any other free fl's they offer or TPR bc yes those tests are kind of crappy, but their good to start off with. Once you are more comfortable with the MCAT type of tests, you should use other resources which are more indicative. Save the AAMC until the very end, as those are the most valuable tests you have and you don't want to waste them. Also just a tip, taking a practice test the first time is really the only time you can be confident in the score it gives you. If you take the same test again a few weeks or even a month later, chances are you will still remember some of the passages and so you might do better on the test the second time around, but that isn't as reliable. But that is just my opinion, others like to take the same exam again a few months, in hopes they forgot about the material but I wouldn't risk that on an AAMC test.
Anywho, don't be discouraged. You will do fine, just get through the content first then start taking any more practice tests. Remember, the MCAT is nothing like any of the other exams you have ever taken, it combines literally everything together so expect for there to be a learning curve. I started out with a 495 on my first practice test, and now I'm averaging a 510 with a 514 as my highest score so far. Don't worry, itll come to you