Hi, Bobbi.
Insufficient info!
Sorry!
Early iPhones (up to the iPhone 6s) have a 3.5mm headphone socket but later models (from iPhone 7 onwards don't). If you've got one of the earlier models, you'll need a cable with a 3.5mm jack plug on the end of it. For a later model, you'll need a wireless adapter.
That only looks at one end of your problem though. The cable from your phone (or wireless adapter, as appropriate) will need to have the correct type of plug(s) to attach it to the amplifier that you're using at the other end. (I'm assuming here that, by '100 watt speaker', you're actually referring to an amplifier that feeds into a speaker. You wouldn't get very far with just a speaker!)
So more information from you is needed before I can advise you any further.
[BTW: If you really are planning on connecting your iPhone to a 100W amplifier, I'm glad that I don't live next door to you! That's heading up towards stadium-filling power!]
;-)