You should deworm at every season change. It's super important to deworm as the worms eat inside the horse's stomach and can cause various troubles... About the dewormer you should use, ask your vet. There are different ones depending on what worms are in your horse.
I recommend getting your horses fecal scoped for a worm count. This will tell you if dewormer is needed st the time he/she is tested. It’s in expensive and worth it. The more unnecessary chemicals you put in your horse, the more problems it can cause later down the road. I haven’t had to worm mine months! The smaller pastures tend to give horses more worms because they tend to have to eat where they have pooped already or other contaminated horses have. Don’t go by the “every month” rule. Go by your horses specific needs. Good luck!
If a horse has worms, you can usually see them in the faeces. Usually, they get wormed using a certain wormer (there are about 5 main types that target different worms) on rotation during certain times of year. However, there's a huge problem of resistance happening where wormers aren't working every time, as the parasites evolve. As a result, people are now more often turning to Faecal Worm Counts which test poo samples for worms. You then get told if your horse has worms, what type and how severe it is. That way you can dose specifically for that worm, rather than routinely dosing, regardless of what worm it may have.
Deworming is the giving of an anthelmintic drug to a human or animal to rid them of helminths parasites, such as roundworm, flukes and tapeworm. You would want to do this as the worms can eat through the internals of the horse thus creating issues for your horse and if severe enough it can kill your horse.
Hope this helps