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Syntax 1

To add a column to an existing table, the ALTER TABLE syntax is:

ALTER TABLE TableName
 ADD ColumnName column-definition;

For example:

ALTER TABLE yourtable
 ADD yourcolumn  varchar2(50);

This will add a column called yourcolumn to the yourtable table.

Syntax 2

To add multiple columns to an existing table, the ALTER TABLE syntax is:

ALTER TABLE TableName
ADD (	column1	column-definition,
	column2	column-definition,
	...
	columnn	column_definition );

For example:

ALTER TABLE yourtable
ADD (	yourcolumn1	varchar2(50),
	yourcolumn2	varchar2(45) );

This will add two columns (yourcolumn1 and yourcolumn2) to the yourtable table.

Get all tables including the system ones:
SELECT table_name FROM all_tables;

That will give you only the tables owned by the user that you are connecting as:
SELECT table_name FROM user_tables;

Query table columns:
SELECT * FROM all_tab_cols WHERE table_name = 'whatever table name you are looking for'

Looking for comments on the tables:
SELECT * FROM all_tab_comments WHERE table_name = 'whatever table name you are looking for'