Both parents can, and do, kill their eggs/fry. Sometimes you have a good daddy that will take care of the fry until they are juveniles, sometimes the dad will eat the eggs.. sometimes a female will stay around and help care for the nest if the male allows her to, sometimes the male will chase her away. So it varies between each pair.
As mentioned, lots and lots of supplies are needed to breed.. breeding tank/s, grow out tank/s, containers to hold all the juveniles that become aggressive after a couple months - depending upon how many you cull (kill) can be from 10 to over a 100 males you will need to house in their own containers - and you would need to be able to keep each one heated appropriately, and you will need to clean out each one daily.. can easily spend a few hours a day just on cleaning alone.
There are cheaper ways to go about it - I personally went a little more expensive as I use actual tanks to breed (have small 1-2g bowls too) as I want to see my fry (eyesight isn't the greatest and want to see the fry to be able to cull the deformities out).
Also keep in mind the electric/water bill will go a lot higher.. I think for my set up I pay over $50 a month in just electric for just the breeding set up - possibly even more, but I'm not in charge of the bills.. I just remember my boyfriend mentioning it one time.
Minimal you are looking at a few hundred dollars to set up a spawn - if you go with used/plastic items.. from there it can go easily into the thousands.
Upkeep costs money..
I would also find out what types you have, fish wise, as you will need to know what to expect when it comes time to sell them and even breed them.. know which types sell for $3, and which ones go upwards of $30. Know how and where to sell them. Know the types you have as you don't want to cross some fin types unless you have a plan and are going to work on generations to reach that goal.
Throwing a female and male together without being properly set up or knowing how to condition them will end up in injury or death. I would do lots of research on those links that were provided to you by Hadoken.. start asking more specific questions in the breeding section of the forum rather than this section. Join groups on Facebook as you will find more breeders there that can help answer questions. IBC is another place you pay to join, but can provide you with good information on how to breed quality fish.
Breeding pet store bettas is fine as long as you are aware that you won't know what colors you will be getting in the fry - if colors aren't a big deal then go for it. Learning while using pet store bettas is a good idea as they are cheap.. better to spend a few dollars on a pair than spend closer to a hundred when you are just learning. But keep in mind again, the type of betta you have - VTs are very common, but you won't be getting many people purchasing them from you whereas you get some nice CTs, HMs, PKs going you will have a better chance at selling them. BUT there is nothing wrong with breeding VTs, just know what you are getting into before breeding them. Not many people are willing to buy the fish and then spend $35 on shipping for a fish they can pick up for $3 at Petco. So research what you are wanting - color (not a must), and fins.. and find out what to look for/not look for in the fish - such as the topline. Optimize your chance at being able to find homes for your fish by researching.
After everything is set up then go for it... you won't know exactly what works best for you and your fish until you actually do it. Just don't attempt it until you are 100% confident you have everything you need, know what to do with the babies once they grow up, how to sell them, etc.
Babies need live food.. lots of cultures out there to grow for newborn fry like micro worms, banana worms, vinegar eels.. baby brine shrimp is also a good one.. a can can run roughly $50-$85.. just have to hatch them yourself daily.
