Check sysobjects and syscolumns tables.
Here is a diagram of Sybase system tables.
List of all user tables:
SELECT * FROM sysobjects WHERE type = 'U'
You can change 'U' to other objects:
List of columns in a table:
SELECT sc.*
FROM syscolumns sc
INNER JOIN sysobjects so ON sc.id = so.id
WHERE so.name = 'my_table_name'