Are they easy to take care? And do I have to put them in brackish water to order to breeding them? Just wondering.....I should buy a book of how to take care of native fishes instead for looking answers in google...

Never bred them, but they are very, very easy keepers. Ate flakes right away.

They eat everything, add some salt and they'll be very happy, doesn't have to be brackish though, just the standard freshwater application of salt. The males really color up for spawning, my daughter asked me the other day when I got the new fish LOL

They are indeed very hardy and easy to keep.
I have never tried to breed them but have seen them breeding many times. They like shallow water with scattered plants. The males chase each other and the females and there's lots of darting around. Very much like banded killies in freshwater.
Although they are said to be able to thrive in fresh water, the only freshwater I ever see them in is in little overwash pools behind the barrier beaches. Mostly fresh water seepage and puddled rainwater, they periodically get inundated with ocean water during big storms. Lacking aquatic preditors and too murky to be easy pickings for birds, mummies [and fourspine sticklebacks] are sometimes found in very dense populations. The dreaded saltmarsh mosquito apparently provides ideal forage.
I guess that's not an answer to your question but maybe it'll set you on the right track.

Not hard to breed at all. If you can't give them saltwater, give them freshwater with a lot of calcium hardness--add a little calcium chloride (sold for melting ice on driveways and sidewalks) or dissolve some garden limestone in the water during water changes. If you don't have naturally hard tap water, the easiest thing to do is add a teaspoon or so of marine salt per gallon. Not aquarium salt. Marine salt. It not only has sodium but also calcium and buffers that keep the pH high. Get one of the less expensive ones, like Instant Ocean. Mummichog don't need the highly purified reef salts.

If you put the temperature up past the mid 70s and give them ten to twelve hours of light, they'll breed. The easiest thing to do is put a pair or trio in a well planted tank and then remove them after a week or two. With luck, at least some of the eggs will have survived and you'll have a hatch. I bred them once, by accident in my 65 gallon tank. The tank had lots of mummichogs and sailfin mollies. Somehow, one egg survived and grew up to be a neat male.

The other thing you can do is keep them in a tank with a lot of yarn spawning mops, and pick the eggs like you would do for any other mop spawning killie. They aren't fussy--they'll spawn in plants, mops, gravel, and even on a bare bottom tank.

Are they easy to take care? And do I have to put them in brackish water to order to breeding them? Just wondering.....I should buy a book of how to take care of native fishes instead for looking answers in google...