You can attack each round.
Sustaining the sphere (a minor action) and attacking with it (a standard action) are two separate actions.
[...] I know that it only needs to be sustained once, [...]
That's not true: in order to keep it active you have to spend a minor action each round. By doing so, you prolong the duration until the end of your next turn. You can sustain the sphere this way up to the end of the encounter (or up to 5 minutes, if not cast during an encounter).
That's both confirmed by the general rules for sustaining (PHB, page 278) and the Flaming Sphere'sDDI Sustain line:
Sustain Minor: The sphere persists until the end of your next turn.
In general, keep in mind that no power must be sustained "only once". D&D 4th edition rationalized power durations, and it's quite difficult to find a power that is not included in one of these categories:
- Until the start/end of your/target's next turn.
- Until the end of the encounter (or 5 minutes, if not in an encounter).
- Save ends: target(s) rolls each round until it shrug off the power's effect (sometimes there are one or more aftereffects).
- Sustainable powers: you must expend an action (usually minor, but it could also be move or standard) each round you want to prolong the power effect for another round.
While Flaming Sphere is active, you can spend a standard action to make another attack with the sphere (the first comes with the same action used to "cast" this power). If, for example, you expend an action point, you can use that standard action to attack with the sphere a second time.