I want to create a debian metapackage with my version of tilda (from my local repository), but I don't have any idea how to do this. I put my metapackage and my version of tilda in my local repository, but I have a problem. How do I specify that the tilda package should be installed from my repository? (That it's must be a metapackage because I want to add some more dependencies in future).
There is no straightforward way for a package to affect what gets installed on a system before it is itself installed. The common ways to manage installation preferences is by either (a) making sure the custom package has a version number which is higher than the regular package's, or (b) for the admin to install a pin on the preferred repo. I would go with the former because it's simple and intuitive, and only requires the admin to enable your repo.
There is no particular reason for you to create a separate metapackage if you are building your own custom version of the tilda package anyway. Just add the other packages you want to pull in to the Depends: part of the package's debian/control file.