I broke Wordpress file and map ownership

Hi all, I hope it is possible to help me with the following case.

Because of the github developer pack I started trying to run some websites and see what I can learn in this field. However I think I broke something which I don’t know how to fix.

I run the one-click-wordpress install. So all is relatively standard. I did set up SSH access with another account so that root is not used.

Now the website itself seems to work, however on the backend I can no longer install updates (core and plugins) or upload files even. They return a similar error that either they cannot create a map or cannot move a file (image for example).

Now I read that ownership permissions for files should be set correctly, according to wordpress they should be 644 for files and 755 for folders. Which is what I set them all at.

But still it seems to not work, so I guess the owner of the folders and files is the wrong one. But I don’t know how to set those or to set those to the right user?

I would really like help on that one to solve the updating through the buildin update mechanics of wordpress (and of course uploading).

Thanks in advance!