Use Fire II on packs of 3 or more monsters like Genoreaper said. Two or less, use single target.

As for Blizzard III, you should be using it to switch to Umbral so you get MP back once you've consumed it all casting Fire spells. BLM generally doesn't want to spend a long time in Umbral, so once you're full MP and have refreshed Thunder, use Fire III to get back to Astral.

There's more to it as you get more skills, but that's probably enough to get you started.