After placing the breeding cave, you simply tap on the cave. It will give you a list. You select two different dragons from the list, and tap breed. After that, you just wait to see what egg was the result.
fire+plant in that order
Plant+fire=poison
Detail about breeding poison dragon in dragonvale: check the link.
Plant + Fire = Poison Dragon
Breed a plant-dragon with a fire-dragon. Make sure that the plant is on the left side when choosing dragons to breed, and fire is on the right.
The most effective way of breeding a current dragon is water dragon (lv 10 left) and a magnetic dragon (lv 9 right). You can also do this with two level 10s, but the success rate drops from about 55% to about 8%.
The current egg is dark blue with a turquoise lightning bolt. It is a "secret" egg, and not available in the market. It takes 16 hours to breed, and another 16 hours to hatch. It becomes available at level 14, and gives 75,000 experience points when hatched.
Other methods of breeding are:
The best way to breed a mine dragon is a Metal Dragon (lv 10 left) and a Cold Dragon (lv 10 right). This has about a 75% success rate.
You have to breed a plant and a fire dragon.
Chrom/scortch, plant/forge, storm/forge, metal/fire, or brass/flower
breed plant and fire
The gold dragon is a permanent legendary dragon, so you can take your time trying to breed it.
You don't breed a Kairos Dragon in DragonVale. You have to get parts to complete it.
You have to either buy it for 500,000 gold, or breed water dragon with another water dragon.
Fire plus metal in the epic breeding cave
you breed a ice dragon with a mine dragon
You have to breed a plant and a fire dragon.
As of July 2012, there has not been a star dragon in DragonVale.
You have to breed a sun and moon dragon together
You have to breed a metal and a water dragon together.
You have to breed a plasma dragon with a salamander dragon.
Breed the earth dragon with the fire dragon.
To get a Smoke Dragon, breed a fire and air dragon together.