Howdy and Welcome!
Indeed there is much to do to become more proficient at breath-hold or apnea as we call it. I suggest that you take a look at the "how to start freediving" thread in this "Beginner Freediving" forum.... Take a look at the thread titled "Tables" as well - I also offer some basic info in "new to freediving..". There is a lot of great info on this site and some of the best divers in the world pop in here quite regularly to answer your questions.
If you are interested in freediving, you might want to aproach this discipline by dividing it up into several different studies. Gas exchange/imbalances, Technique and Technical Freediving, Physiology and Physics of Freediving, and Psychology of Freediving. There is serious, "must have" information in all those areas for the serious deep swimmer.
Regarding your question: Basically, there are a bunch of different kinds of breath hold ex. One of the best is the C02/02 Tolerance Tables. There has been a lot of discussion on this topic by many folks on these forums. If you do a search on our profiles you can see what threads we have been active on and read the topics related to your questions. Diligent practice of tolerance tables will give you spectacular results.
Most important is the basic breath-up.... and there are other techniques related to fine tuning gas balances that should be taught later with personal instruction.
You have a lot of different things that you can do in the way of pool training and other activities that can be done hypoxically.... just swim laps breathing every other side every other stroke, or every third or fifth or seventh stroke. do that for 500yds a workout.
One shouldn't do breath hold practice in a pool solo. The lifeguard is not watching you and blackouts can happen quite suddenly and without warning. However, one lap crawl, one lap kicking. one lap underwater is good...
If you are planning on learning to be a deep swimmer I suggest that you look all over this forum and that you consider a course from our hosts here at deeperblue or, if you are stateside,
www.performancefreediving.com
Also, there are opportunities on this board to go and do training. I have a house in Kona where I am inviting all kinds of divers to come and visit. Performance Freediving holds courses in Kona as well...
So welcome aboard, chat it up. If you want to PM me or want to ask specific questions, go ahead. Its about community and safety. We want you to live through your interest in this discipline.
Lungfish