Welcome to UCSD! I was in your position 4 years ago and also asked others about this school on SDN.
Like someone mentioned above me, there's a lot of opportunities for you to succeed as a premed. It's good that you're planning ahead so that you know exactly what to do --taking courses in prep for MCAT, application cycles, extracurriculars, etc. From my experience, the weeder courses aren't particularly hard, except for CHEM 140B. One tip I always tell others is to take the ochem series off sequence, meaning CHEM 140A aim winter quarter, CHEM 140B spring, etc. The standards are a lot lower. Took CHEM 140A and CHEM 140B along with everyone else in fall and winter, and they were some of the harder weeder classes at UCSD. Took CHEM 140C in spring, dropped to take a W after receiving my midterm grade, and retook next winter quarter to get an A+. And trust me when I say that the standards are different because I used my material from the quarter I dropped to prep and I couldn't even finish his practice midterm. So yeah! Start the series in winter quarter because I've had a handful of friends who had to drop CHEM 140A in fall after failing their midterms only to take CHEM 140A again in winter --their midterms and finals are cake.
But I think what's more difficult is managing all your classes along with your extracurriculars, especially as a freshman when you're trying to make new friends. I knew I wanted to apply the cycle before senior year, so I joined a lot of clubs to figure out which one was worthwhile. Know what you're able to handle or your GPA will tank! I know A LOT of my friends who's GPAs were dragged down because of their freshman year. Don't be afraid to take 12 units when others are taking like 20, lol. Good luck! PM me if you have specific questions.
Edit: if you think you're up for it, you can go on port triton and look for a lab position now before you start freshman year. Even if you haven't taken any classes (which doesn't even really matter in my experience), they'll like you more than current 2nd or 3rd years if you say that you'll be staying for 3+ years.