Brave new Challenge you're picking up! LOL.
Ok, The key is relaxation.
A good source is
FreeDiving, Scuba Diving, Spearfishing & Technical Diving Articles and News - DeeperBlue.com , Where on their forum are some very knowledgeable freedivers, including World Record holders such as Dave Mullins who are very generous in giving the best tips to almost anyone who asks.
In order to train safely, explore safely your aquatic potential it's best to have an educated, experienced freedive buddy. I'm sure that you may find someone not to far away through deeperblue or your American Freediving association. If you have the cash I recommend saving up for an freediving course, AIDA - FIT - Apnea Academy. Trough such a course you'll learn the skills that otherwise may take a few years to find out by trial and error. The knowledge that will help you to progress and have fun discovering and exploring the amazing aquatic abilities we have.
Things you already can do.
Dry land challenges:
Apnea walking, walking over soft soil or grass, holding one's breath. The goal is to learn to relax through the increase in CO2, move efficiently, exploring different preparations, get used to lactic acid forming, focus, mind control.
Apnea on a couch / bed.
Pure training of the body and mind. There are schedules you can use daily, improving your breath hold amazingly over the course of weeks.
You'll find that in the beginning just a 30 sec is difficult, after a week you'll do with relative ease 2+, maybe even 3+, of which the first minute is pure bliss and relaxation.
Later you'll find that the first two minutes are very comfortable.
In the pool you should have a spotter, who knows how to rescue a person who blacks outs under water. The Lifeguard knowledge can be very limited. Besides he needs to watch EVERYONE. Most pools forbid freediving, because of the perceived risks.
But you are alone and still want to enjoy the underwater freediving? Well you can if you are honest, do not hyperventilate and limit yourself to coming up after the first urge to breath. Avoid Hyperventilation, breathing more than you naturally need, as this shifts your 'alarm bell' - maybe beyond your O2 supply. Instead you'll find that Hydrodynamics, relaxation, rhythm, concentration, swimming technique, and getting used to more CO2 will extend your dive gradually to previously perceived impossible lengths.
Don't forget the fun factor, think of fun things to do, ways to move, fly, sink, be, feel.
You'll discover also the impact of nutrition and hydration. <- very important.
There are basically two apnea schedules, that are used to train: CO2 tolerance and Hypoxia. Table A and B.
Table A (CO2), uses the same base time (~ 0.5 of your personal best), and decreases the breathing interval times.
Table B (O2), uses the same breathing interval time (2 minutes) and increases the breath-hold time to 90% maximum of your personal best.
Table A is relatively safe to do swimming in a pool alone, where Table B is not.
You can do Table B on the couch and in bed or on soft soil (in case you push yourself too far you'll land relative softly

)
Through table A you'll learn how more CO2 feels, as due to shorter interval times more is retained in the body. The O2 is very quickly restored, usually with just 2 breaths, hence you see why this is relatively safe.
Other training on land can be things like:
- concentration games, focussing only on your food while eating. Counting 0 - 10 - without any interruption of thoughts. (you've guessed it right, there are books written about just this ability!)
- Meditation,
- Qi Gong, martial arts.
- Flexibility (chest, diaphragm, Eustachian tubes)
- General fitness, adding some muscle mass if you're skinny.
- breathing exercises - prana yama (You will learn the power of breathing)
- visualisation.
You see, it's a very complete sport, and it can become a lifestyle. You'll become so much more conscious of our body and mind. Through this training of the senses, the increased bloodflow to the brain, and the cleansing (Detox) effect freediving has on the ability to let you percieve new levels of colours in our world. I remember for instance That I started noticing all the delegate smells in spring. Also the richness in other area's like sound and vision increased.
Anyway I should quit now, otherwise I'll end up typing a whole book on the passion that is responsible for a great deal of my development.
Love, Courage and Water,
Kars
ps. start slow, take your time, and ONLY do small personal bests. It's easy to over train in freediving, and damage something that can be pure bliss.