How can I find out someone else's IP address?

I would like to know how to find out the IP address of others (through messages, …). If it is possible that I can do this with my laptop at all, would that also be legal?

That depends on how or via what you communicate, but yes, in principle it is possible, since the IP is essential for communication and is sent openly.

But I don't know what you want to do with the IP…