1. set undo_management=MANUAL in your init file.
2. connect "/ as sysdba" and startup the database using this init file (if not it will use the SPFILE by default), e.g.:
3. drop the ('undo') tablespace if you wrongly created one without actually specifying "undo"
4. create a new undo tablespace, e.g.:
5. shutdown the database
6. change UNDO_MANAGEMENT=AUTO in your init file and set UNDO_TABLESPACE=UNDOTBS (or whatever name you specified during the undo tablespace creation)
7. connect "/ as sysdba" and startup the database using this init file (as in step 2).
Now the database will open using the new undo tablespace and it would have created new undo segments in this tablespace.