In a 15 gallon, you'd want to have a strong filter, you want it to pump at least 30 gph.
Plants suitable for goldfish are Java Fern (but they grow really big for a 15 gallon, so you'll need to separate them and take some out later as they grow), Dwarf Anubias make a lovely little carpet of leaves if they can grow too
I have two chunks of wood in my tank, just because it's so beautiful, but I wouldn't put it into such a small tank since the fish will want space to swim too! A few small pebbles would be nice too. Simple, and just enough for the fish to be comfortable!
Feed the goldfish some good quality staple pellets, flakes are harder to eat, and pellets come in many different sizes, so buy some smaller pellets in the start! Like Hikari Staple Goldfish pellets or color enhancing Hikari Goldfish Gold (If you want specific brands). They'll also need protein, so get some freeze-dried blood worms, tubifex and/or shrimp, and feed them these a few times a week. As for vegetables, growing some algae in the tank works, but you can also feed them peas, after you've popped them out of their shells, cucumber, zucchini, spinach...
In a 25 gallon, I'd keep only one common goldfish. They grow really big, and they are fast swimmers. In a 15 gallon, I'd at most put a fantail, since they don't grow very big and they're not too fast when it comes to swimming.
Telling the gender of goldfish is something you should wait with until you've kept them for a while. It's very hard to notice the subtle difference and since all goldfish can have such different body-shapes it's not a safe bet to go for the "females are chubbier". Because they're not until they fill with eggs during breeding. Males get white dots on the gill covers and pectoral fins when they're ready to breed.
The best way to note the difference if they're not in breeding-mood is to look at their vents, males have a slightly convex, oval-shaped vent, while females have a round, protruding vent.
You can't see that until the fish are a few years old, since it can take a goldfish between 1-10 years to reach sexual maturity.