Could you tell me the ruling on caffeine in Islam? Is it forbidden to drink coffee or tea? What about coke or other fizzy drinks?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

There is no harm in drinking tea or coffee regardless of the substances they contain. Imam Shihab al-Din said: 'it is lawful to drink because originally all the things are lawful except that which Allah has made unlawful. Allah Says (interpretation of meaning): {Say (O Muhammad SAW): "I find not in that which has been inspired to me anything forbidden …} [6: 145].
So, since coffee does not contain anything that causes drunkenness or any drugs and since it does not harm the body, so it is lawful and cannot be compared to Haram things that cause drunkenness or dizziness.
In fact, coffee makes the body more active. So, the person - after drinking coffee - becomes more able to do some kinds of work. If these works are lawful or if they are forms of worship, then coffee is in this case good as it helps the person do that which is good.
What is said about coffee and tea is true for soft drinks.
On the other hand, you should bear in mind that many of the lawful things can be harmful to the body if over-used.
Allah knows best.