Welcome. Before I started freediving I was in the same boat as you in terms of experience. Been snorkeling/swimming all my life but never any deeper than 30 feet. There is a very good series of 20 articles on DeeperBlue that details most everything you'll need to know starting out.
https://www.deeperblue.com/category/freediving/the-beginners-guide-to-freediving/
Otherwise I can give you a few thoughts. I am no OG by any stretch of the imagination, but I've picked up a few things. One important thing to be aware of is that there are a lot of different opinions on how to do things with freediving. We are just beginning to learn about the physiology of freediving and deep diving so you will often see even veterans of the sport advocating very different techniques. My advice to you is to learn what you can online but take it all with a grain of salt and get into the class where they will teach you most importantly how to dive safely (and yes they usually go to around 60 feet if you're comfortable). Slow and steady wins the race in freediving. There are no magic tricks that will instantly make you better (with the possible exception of relaxation).
You've got the first part with the cardio. Heart and lung health are paramount to what we do so cardio is important. The second part will be to do some apnea specific training. There is debate on the merits of static breath holding but I think it's good to do so you can learn to relax and get comfortable with the sensations. Look up CO2 tables and O2 tables. Do them 2x a week each and if you're wheezing and panting after finishing a breath hold shorten the time to where is challenging but not desperate. You want to train you body to be calm and relaxed even when under stress. You can absolutely overtrain breath holding just like with any other physical activity. Dry dynamic apnea aka: walking apnea or stationary bike apnea is useful because it simulates kicking while holding your breath. Again, challenging not wheezing, since passing out while walking or in the gym can lead to bad things like less freediving for a while.
Personally I find the alkaline apnea specific diet a sham. At best it is beneficial only to professional level athletes and has little relevance recreational freedivers. At worst it is dietarily equivalent to hyperventilation. That said, cleaning up your diet to be healthier is never a bad thing (ie: more veggies, less soda and cookies). Paleo is solid, not necessary and pretty expensive but keeps you with plenty of vitamins and minerals and all of the macronutrients you need. Excessive caffeine (or stimulants) will make your heart rate be higher than necessary and dairy causes increased mucus production which can make equalization trickier. You don't have to get rid of them, but no/light caffeine and no/light dairy on dive days can make the dive more pleasant/successful.
If you start working on those things a little each week you will be more than ready when you reach the class without ever getting into the water. In the 30 to 60 feet range most of the people have trouble going deeper either due to problems equalizing or because it scares them, not breath hold time.
Two final things: 1) Have fun with it. Don't sweat the numbers, be consistent, and you will get better. 2)
If you are in the water doing any sort of breath holding activities bring a buddy that you trust. This is not optional.
TLDR: Relaxation is everything don't force it. Enjoy the process and take it slow, it will come. Always have a buddy in the water.