While I won't be so blunt as Broadzilla (see above), I will tell you that the pipeline IS difficult. If you can't hack it, we'll figure it out before you get to the fleet.
The attrition rate isn't as high as it was back in the 90's (way before I joined the Navy after 9/11), but it definitely isn't easygoing nowadays.
I joined during my third semester in college (majoring in English), and had also only taken precal. I picked up a book called "Basic Physics" by Karl Kuhn and studied it for a week prior to my nuke test. I passed just fine and shipped out a few weeks later to Great Lakes.
The program is hard because information is taught to you at a pace like none other you've experienced before. Each individual topic isn't extremely difficult per se, but add all those topics up into a big comprehensive final exam, and it can be blistering.
Some classes you should look into for the next school year would be physics, chemistry, anything to do with basic electricity/electronics, precal/college algebra/trigonometry (if you didn't learn it well back in HS), and even English. If you think you DO have precal done, take calculus. It will be useful if you want to get a degree in engineering and use it after the navy in six years.
I'm currently instructing at prototype in NY (actually, I'm something called a "Training Coordinator," which involves not so much instructing but more administrative duties and ensuring military rules are being followed by the students), but I'd say (for the most part) instructors work hard to get you trained. That IS their job, after all. You'll get the necessary amount of "motivation" to help you through.
Send me a PM with any questions or email me directly.
Jim
EM1(SS)