definately seahorse.org

there are a number of articles in the library that will help you get a better understanding of their care.
things to consider: will your tank have a chiller? temperature is very important so make sure the species you ultimately choose is correct for your intended tank (seahorses are grouped as temperate, tropical, and subtropical). i currently have two h. comes and they're kept at 73, i try to keep my erectus below 77.
know what size you'd like as well, some like h. ingens can reach 12". erectus are generally considered a good beginner horse, as are kuda. my number one piece of advice is to only purchase CB from a reputable breeder. i have not purchased horses from dan, but i've purchased a number of meds and other supplies from him and would recommend him in a heartbeat. you're going to pay a little more for his horses but it's worth it, he's always available to answer any and all questions regarding care and treatment. his site is seahorsesource.com. steer clear of ANY horse at the LFS until you have a little experience with them, you'll probably see some labelled "tank raised" - i call them WC and while it is possible to keep them successfully they generally require quite a bit of work to get healthy.
have some recommended meds on hand first, if you currently have a tank you may already have some of them - at a minimum i would have formalin, a topical (i.e. hikari's biobandage or neosporin), an all around antibiotic (neomycin/triple sulfa, gentamicin, etc.), diamox would be nice but if you ever need that, i've got plenty. if you do decide to purchase from a LFS i would also round up some praziquantel or prazipro and fenbendazole, depending on where the horse was purchased and how they were raised (net penned in sri lanka, vietnam, etc.) they can come in rather wormy and with a high parasite load. i generally perform a freshwater dip or formalin bath prior to placing in QT.
while there is still a lot to learn about their care, there is a good deal of information available to us hobbyists. necroscopies are routinely performed on member horses by labdoc over at seahorse.org (he performed one for me) so the knowledge base is ever expanding.
i don't want to scare you off

but i commend you for doing your research prior to purchase.
QT ! even if you purchase from a reputable breeder.