how can i use tethering with my iphone?

What model iPhone, what country, and what carrier?

Also, see this tech note from Apple: iPhone: System requirements for Personal Hotspot

Tethering/HotSpot has to be activated in your data plan with your carrier. It almost always costs extra.

According to this document, T-Mobile in the Netherlands does not support Personal Hotspot, only Vodafone does, thus that setting will not appear for you.

http://support.apple.com/kb/HT1937

Does T-Mobile in the Netherlands offer a tethering plan that includes the iPhone?

You need to determine that first.

You might call T-Mobile. The Apple document isn't always up to date or 100% accurate.