The rule number one: always freedive with a buddy. Clay-doh is right.Freediving or freediving-spearfishing is probably the most dangerous sport activity in the world. There is something between 30-70 fatalities worldwide involved in freediving every year.
Relaxation is one of the most important things in Freediving, if you're not relaxed you won't be able to Freedive well.
Proper breath up or pre-dive relaxation techniques as yoga is what experienced freedivers do.
Shallow water blackout is responsible for most freediving deaths. Usually occurs in 30ft of water or less and primary contributing factors are: hyperventilation, exercise, a competitive personality and youth. How to avoid it? Don't hyperventilate,don't push your limits,take a rest between dives:for dives above 60ft 2x the duration of the previous dive and for dives below 60ft at least 2.5x the duration of the previous dive. So if you freedive to 50ft in total dive time of 1:30' your surface interval should be at least 3:00' before you dive again.
Practice a safety procedures with your buddy.
Increase your depth gradually. Your buddy should meet you on your ascent and swim up close to you at least the last 30ft(most dangerous zone for SWB) He needs to be ready bring you to the surface in case of SWB. You should be also properly weighted-neutral at 30ft(if you diving deep)
Dry apnea,walk apnea,static apnea CO2 tables,O2 tables,practicing in the pool is what I'm doing to increase my breathold.
There is too much to say and explain about "how to start freediving" and it's definitely not "JUST JUMP OVERBOARD"
Best thing you can do is take a freediving course
http://www.freedivinginstructors.com/ or
http://www.performancefreediving.com/
if you can't afford it there is a great book "MANUAL OF FREEDIVING" by Umberto Pelizare you can find it on internet. It gives you a lot of answers.
Or post some questions here I can try answer them.( I got both,the book and course as well,ha ha)
Good luck and freedive safe