Depends on what you want to breed.
Easy ones that require no expertise like Livebearers - Mollys, Guppys etc or ones that require a huge amount of expertise?
To sex Mollies/Guppys is easier than most.
The fin under their body nearest their tail is different.
Male is pointed and thin. It points back towards the tail
Females is triangular and not pointed.
Then you have the problem of the fry. There will be between 40 and 200 every month or so and they are hard to get rid of as shops don't like taking a fish they can only sell for a couple of quid!!
I used to have some mollys but also had some predators which would scoop up the fry when they felt like a snack.
andy