But I don't think I get it right, does it cost anything to run a node? i mean you do that on an average normal computer no?
Yes, building a decent node costs at least 90 dollars , and has a maintenance fee for hosting the bandwidth and electricity. Thus ideally a LN channel node should exceed 15 dollars a month to overcome this in tx fees revenue.
Ok correct me if I'm wrong but centralization of nodes could get some power to the one running them no? Hence decentralization seems a good thing, but if anyone wants to centralize the nodes of the LN, he will only need to put his nodes for free! Then nobody else will use the paying nodes no?
There will be many different competing models of nodes and their features. Some can act as escrow agents with higher level services and insurance programs, some will offer better anonymity through coinjoin/CT/mixing, some will merely have an advantage as they represent a unique hop for better connectivity between other nodes.
I wouldn't worry about free nodes as their are few miners who process free tx right now ... Free LN channels would quickly get filled with spam and people would use other channels.