Oracle newbie--opening database and listing tables?

Im new to Oracle but experienced in Sybase. What is the command in Oracle to (these
are all from the SQL server command prompt):

1. List databases from the SQL prompt?
2. Use a particular database?
3. List tables and their definitions?
4. Export data from a table (join) to an ascii output file?

Im good to go w/SQL but cant seem to do the above like I could in Sybase, Just need the commands as I have no documentation at all.

TIA,
Martin Received on Tue Aug 04 1998 - 19:51:26 CDT