It is impossible to know this. We don't even know for sure if Iran is trying to get a nuclear weapon. Israel certainly thinks so, but the latest US intelligence report claims that Iran is not.
As to what happens if Iran gets a weapon, it is not at all clear. It is possible that Iran will use nuclear weapons to attack Israel as some people fear. It is also possible that Iran will use the weapons as a bargaining tool in the way that North Korea does. It might use the threat of its nuclear weapons as a way to force the international community to give it aid or to allow it to have its way in various international disputes. No one can know this for sure, but my guess is that it is most likely to use nuclear weapons in this second way.