Skittles pretty much covered the most important parts.
Personally I prefer a glass aquarium with a light built into the hood. It's not really better or worse then a plastic like you have, it just looks nicer. You can also get a plastic aquarium with a light built into the hood now a days though, which may be cheaper.
You can calculate the volume of the tank by multiplying the length with the hight and the depth. This will give you cubic centimeters, which you can convert to liters, which you can convert to gallons. here's a nice little tool
http://www.hoptechno.com/nightcrew/sante7000/convert.cfmI'm sorry I can't think of an easier way, but I've never measured anything in inches.
I think guppies are lovely starter fish, and having a lot of fry is often a sure sign you are doing something right. If you want to avoid having fry you can buy only males though.
As Skittles said bettas are wonderful fish too, they also are quite bright; I managed to teach mine to swim through hoops and jump on a plastic floater for food. No other fish managed that till now. It just takes a lot of patience.
Goldfish are really nice too but they need a lot of room and a good filter. Carp is an anagram for crap for a reason. But in my experience these are by far the most affectionate fish and since they have a long lifespan you can really create a bond.
If I were you I'd buy new fishfood and probably a new dechlorinator. Especially if the bottle is a few years old.
Also, depending on the temperature in your house you may need a heater if you want to keep guppies or bettas.