You can go to the advanced search (the magnifying glass). Then click on the filter tab.
Make a filter for the kind of things you want to delete. For instance, to delete all journal entries:
[tag[Journal]]
then click on the little trash can icon to delete all Journal entries.
Likewise for table of contents
[tag[TableOfContents]]
It may take awhile to figure out which things you want to delete. You can't delete everything -- you might delete the context search feature!
You can also search everything that is recent:
[!sort[created]!is[system]limit[25]]
This shows the 25 most recent tiddlers that are not system tiddlers.
You can check this list, and if it's nothing you need, delete again. Change 25 to whatever number you need to prevent deleting something useful.
Hope that helped.