Most often when you want to say that you are undecided the verb estar is used + indeciso/indecisa. For example, José está indeciso.. Joe is undecided or can’t decide.

However, there are a few Spanish idioms which express the same idea. Since the language is very idiomatic it is best to learn these forms in case you hear them used by a native Spanish speaker.

Estar entre dos aguas – to be between two waters
Estar en la cerca – to be sitting on the fence
Estar con un pie adentro y otro afuera – to have one foot inside and one foot outside