Seahorses can be very difficult and expensive to keep. They need a heated marine tank with filter system but very light flow.
Because they are very specific you will normally be advised to keep them seperate from most types of fish. Pipeworms or small community fish with similar needs are all that can be kept with them.
Their feeding habbits can be very difficult to maintain. Most species require feeding 3 or 4 times daily.
Have you thought about setting up a community marine tank instead? This can be much easier and just as rewarding. Clown fish, royal grammas and firefish are particularly attractive and normally on the hardier side!