lotus notes database is full of proprietary design components. Assuming you just want to export the data, you will need to write an agent, (aka batch process), that would look at all documents in the database and then export all the fields into a plain text file.
If you're into XML, you can export data in that format as well, but again, you will need to write an agent for that.
Alternatively, there are some basic builtin mechanisms in Lotus Notes to export data but this is restricted to running them from views. Views do no necessarily get all the documents. You can design a view to do that though.
Providing the size of the database is not extremely large, (less than 200k documents), you can create view listing all the columns you want to export and a view formula that has "Select @All", will give you all documents. Then, the "quickest" way to get data out from a view can be found here using simple export procedure.
There is still the issues of exporting rich media, you can have a look here for that.
