Hi _underwater_mermaid_
First, you need to work on keeping the fish you have alive. Then we can work on helping you to breeding them.
Why not take a few minutes to fill our your profile? There is a space where you can write in information about the size of your tank and the fish you have. If you list your location, it would help too, just in case someone wants to tell you about medication or aquarium products that are different from one country to another.
For starters, what kind of fish do you have and what is the problem with them? Do you test your water for ammonia, nitrates and nitrites? How long has your tank been running and what is the temperature?
Now, to answer your question, corys are schooling fish which means they should be kept in a group of three or more. I like to buy 6 or so at the same time from the lfs. I usually ask for the 3 biggest ones which will probably turn out to be females and 3 smaller ones which might, but not necessarily, turn out to be males. At least the odds of getting some fish of both genders is very good.
A far as keeping the fry in a breeding net, I don't recommend it.
The fry of any egg laying fish are too small and fragile for that.