I am ready to do some secondary brewing in a glass carboy and wonder if you have to add anything to the beer in the secondary container. Sugar, more yeast etc.
No. You don't HAVE to.
Generally speaking, all of your sugars should have been added at boiling time.
Some imperial style beers have a very high original gravity and attain alcohol levels above the survival levels of the initial yeast population. In this case, more yeast of a different strain (i.e. champagne yeast) can be added to continue fermenting the available sugars. If you feel you have a stuck fermentation, then more yeast could be added to revitalize the fermentation.
If your recipe calls for dry hopping, fruit additions, coffee, penguin beaks, or anything of this sort, then after racking to the secondary is a good time for these additions. But plenty of tasty beers are made without any additions to the secondary.