Once you're working within Lightroom's library, there are many ways you can organize and work with the photos. In my opinion, the best way is with metadata: apply keywords/tags to your photos, as well as location, star ratings, flags, etc. I think the biggest trick is to figure out which of the many metadata capabilities are of interest to you and how to best make use of them.

In the above description, notice something missing: sorting with a folder structure is unnecessary because of all of the better methods available. You can even use a "date" filter to get at the photos you want based on date. Combine that with a location and keyword or two, and you'll have an easy time getting to the photos you want without ever having to think about the folder structure or where your photo actually appears. IMO, this is one of the core benefits of a database-backed library tool like Lightroom!

Starting now you can, of course, import photos into a date-based folder structure that works for you. You don't need to go back to the existing photos to move them. For organizations sake, it is pretty easy to do, however:

1. Use the filter bar (Library menu > Filter by Metadata > Enable)
2. Set a column to the "Date" field and pick a year (and month and day, if you want). You should see the filtered results.
3. Go to the Folders panel in the left sidebar and find/create the folder you want.
4. Select and drag the filtered photos to the folder.

Doing this within Lightroom allows Lightroom to track the move and keep it's database updated.