Rikku knows four basic (level 1) elemental spells: Fire, Blizzard, Water and Thunder. These are offensive/attack spells, but weak ones - less damage then punching someone, usually. The amount of water generated from "Water" has been mentioned in FH canon as being around a bucket's worth, "Blizzard" is that amount of ice, "Fire" is a small fireball, and "Thunder" is a weak lightning bolt.
Rikku also learned a low level White Magic/Healing spell, Esuna, from Lulu -- this can remove status effects (Poison, Silence, Petrification).
Rikku doesn't need any preparation time or items to cast these spells, but she would wear herself out if she tried to cast, say, ten times in a row, and would need rest before she was able to do more magic.
In her canon, anyone can learn magic, so she could teach someone else these spells if their background allowed for it.
Re: Magic abilities
Rikku also learned a low level White Magic/Healing spell, Esuna, from Lulu -- this can remove status effects (Poison, Silence, Petrification).
Rikku doesn't need any preparation time or items to cast these spells, but she would wear herself out if she tried to cast, say, ten times in a row, and would need rest before she was able to do more magic.
In her canon, anyone can learn magic, so she could teach someone else these spells if their background allowed for it.