At the moment nextcloud sychronised with my computer. That means I have data on nextcloud and on the computer. Now my data storage at my computer is full. Thats why I would like to have the data storage on next cloud only. How am I supposed to do that?
Hello and welcome to the community support forum 
You can either mount your Nextcloud via WebDAV (then you can only open your files if you are connected to the internet) or you use the Virtual Drive feature which similar to dropbox keeps opened files around for some time but does not download and synchronize all files.
@Elena hey… just chimed in to drop you a short line…
as much as I personally understand your wish to try and persuade someone to setup your system correctly for you this isn’t the right way in terms of a community-idea our forum is based upon.
I mean even if you would be able to get your problems solved this way what would be the benefit for the forum and it’s users?
So the common way to get your problems solved is: get into things here on the forum and let the audience be part of how you’d get your problem solved… so that someone could follow the directions in case of having similar problems. OK?
Awww… and posting your private data openly on a public forum is never a good idea. NEVER!