Question: I need to have a homepage running on my Mac that external users can use. I thought the easiest way, was to make Apache Tomcat running on port 80. But when i try it says that the port are in use.

I can't see anything that are using port 80. 

What to do???

Answer: 
Since Tomcat is listening on port 80, you need to start it up using sudo. For example: `sudo catalina.sh start`

(I already answered this in the comment above but figured I should submit an answer for completeness)
OSX comes with a built-in Apache, which is probably enabled.

Got to "system preferences"->"sharing"->"file & web"->"Web sharing"

Make sure web sharing is disabled.