I've been trying to get into plugin development and didn't find many usefull documentation. 
I searched the net for any info on this and found BasePlayer.FindPlayerById and BasePlayer.FindPlayer but none of these seemed to work.
I'm sure this has a simple sollution.
(Sorry if I posted this in the wrong thread)

Those are methods provided by the game, not Oxide. Both of those would work though to get the BasePlayer, but that's only useful if they are online as a BasePlayer wouldn't exist otherwise.

I found the sollution to my problem. I had to use this.covalence.Players.FindPlayer because the plugin I was working on was a RustPlugin. This function gave me an IPlayer, and I could acces it's name with .Name. As far as I understand this only works with players who have ever connected to your server. 

Yes, an IPlayer would only exist if they have connected before and the .data file exists with them in it.