Open SQL Developer, connect to the database.
Below i connected to SYS user and then go to Other User in left pane. Here i am going to take the DDL of EMP table from SCOTT Schema Expand it and select the table for which you are looking for DDL..
Click on Save to File option and select the path for this DDL and your are done.
We are done with the DDL of EMP.
Hope it helps.