Well, you'd need a tank somewheres around 15 gallons, some live rock, some flowrate from powerheads, heater, basic strip lighting, some salt mix, water, test it, food, and of course the fish.
If I may ask, why are you considering getting your G/F a clownfish pair for her bday? Is this something she's always wanted, or what? I only ask cause sometimes giving pets as presents isn't the best idea if the recipient is unable/unwilling to care for the gift.
You might want to consider cheaper "Nano Cube" type tanks as they come with basic lighting, powerheads, heaters, stand, hood, etc allready pre-made at a decent price. They make good Fish Only tanks
