"Which chair will you sit on?"

Translation: Melyik székre ülsz?

[Ka.Nie]

To indicate future "will sit" I would use leüülni Okay?

[Findil]

"Melyik székre ülsz le?" would be just as correct. This verb in present tense can indicate future with or without the prefix as well.

[jzsuzsi]

Melyik székre fogsz leülni? Melyik széken fogsz ülni? these are also good for future tense.

[NotALandlord]

Would széken also work for on the chair?

[mcfly17]

Zsuzsi is correct. Some would also use "majd" to indicate some undetermined time in the future, as in "Majd melyik székre ülsz?"