it depends on whether you want a fish tank or a coral reef tank.
Get a tank that is as big as possible. The more water you have the more stabile the environment and the more variety of fish you can keep.
If you want fish and shrimp but no corals then an external power filter is probably fine. Get one that can do the tank volume 10 times an hour or more. Have a single aquarium light and a heater. A protien skimmer can help keep the water cleaner for longer and is useful but not essential. You will need a hydrometer or refractometer to measure the salinity (salt level). You will need some sand or shell for the bottom of the tank, and some pieces of liestone rock for decoration.
A coral tank will require better lighting and live rock in the tank. Live rock is rock from the ocean and is used to build up a ledge in the tank for the corals to sit on. It is also used to help filter the water. You can use an external power filter, a trickle filter, or the live rock to filter a coral tank.
You will need several power heads to provide water movement. Coral tanks have huge amounts of water movement, often the pumps will turn over the tank volume 30 or more times per hour.
You normally test the salinity a couple of times a week, depending on if you have coverglass or the tank is open on top. An open top tank should be tested every day and freshwater added when necessary to prevent the salinity from fluctuating.
In a fish only tank the Ammonia, nitrite & nitrate can be tested once or twice a week.
If you have a coral tank then test for phosphate, calcium & KH (carbonate hardness) as well.
