To get started in aquariums, you'll need to know a few things. I don't know how much you know, so I just wrote it all down. I'll add links later:
1: Cycling- This refers to the process by which waste from the fish is processed by bacteria in the filter into less harmful chemicals.
2: Filtration- Most tanks need a filter, and there are several diffferent kinds. For a tank your size, you'll probably want a power filter. Filters circulate water and catch particles that are suspended in the water, keeping the water clean. More importantly, they provide a place for beneficial bacteria to grow. If you get a filter, make sure it isn't one of those that tells you to change the pad. What you want is a filter with a reusable pad or sponge that you merely rinse out when it gets dirty. Make sure you never clean out the filter with tap water, use old tank water from a water change.
This article explains both cycling and filtration: http://fins.actwin.com/mirror/filters.html
3: Water conditions- this refers to the chemical composition of the water in which the fish are kept. pH refers to how acidic or basic the water is, GH and KH refer to the hardness and buffering capacity of the water. "Buffering" refers to the water's resistance to rapid changes in pH, which are stressful to fish.
The first part of this article goes into more detail: http://www.chelonia.org/articles/waterchemistry.htm
4: Diseases- there are many, many diseases fish can get, but the most common ones are Ich, Velvet, Fin Rot, Columnaris (mouth or body "fungus"), and internal parasites. There are many more than that, but those seem to be the most common. Some diseases are always in the tank, but they don't affect the fish unless it is stressed, so the best way to prevent them is to keep the water clean. Other diseases (usually parasites) can only be introduced by an infected fish, so the best way to prevent them is to quarantine fish for a few weeks before adding them to an established tank.
Here's a good link: http://badmanstropicalfish.com/fish_...ification.html
5: Cleaning the Tank- Cleaning the tank involves removing solid fish waste (called "mulm") and chemical fish waste (ammonia, nitrite, and nitrate; see Cycling). There are three main actions for cleaning the tank: water changes, which is when water is taken out of the tank and replaced with fresh water; vacuuming, which is when a hose is used to suck mulm from the substrate and remove it from the tank; and filter cleaning, which is when mulm is rinsed from filter pads.
Planted aquariums are a specialty type of aquarium that have additional requirements:
1: Light- Most aquariums only need a light for the purpose of making it easier to see the fish. Most plants require more light than that, and in particular shades. The amount of light determines how quickly a plant will grow, but if the light is too low, some plants will die.
2: CO2- Carbon is a basic building block of plant cells, and they can't grow without it. In all aquariums, there is some CO2, but it is quickly used up by most plants. The lower the light, the less the demand for CO2, but most plants will benefit from additional CO2. There are three ways to add carbon to the tank: DIY CO2, which is when yeast is used to produce CO2 and add it to the tank; Excel, which is a chemical which can be added to the tank as a liquid source of carbon; and pressurized CO2, which is when bottled CO2 is purchased and injected into the tank.
3: Fertilizers- All plants need certain nutrients in addition to CO2 to grow. These include macro nutrients -nitrogen, phosphorus, and potassium- which are required in large amounts, and micro nutrients -iron and others- which are required in smaller amounts. Fertilizers can be added to the tank in liquid or powdered form, or can be added to the substrate.
4: Substrate- Most plants have roots and need something in which to grow. There is a wide variety of substrates you can use, including sand, gravel, dirt, and commercially available substrates, like Flourite or Aquasoil. An important aspect of the substrate is carbon exchange capacity (CEC), which is the substrate's ability to adsorb nutrients and hold onto them until the plants need them.
These are just the basics, but once you learn about these you should be able to handle finding more info without feeling overwhelmed. Remember, this is a hobby, and it is your tank, so if you're not having fun, there's not much point, no matter how healthy your fish are. It's okay not to have the ideal setup, and it's okay to make mistakes and have tank crashes along the way.