In Oracle you should define a sequence in your database schema e.g.
CREATE SEQUENCE emp_id_seq;
Then you can get the next value off this sequence in your SQL INSERT statement:
INSERT INTO employee (id, firstname, ...) VALUES(emp_id_seq.NEXTVAL, ?,....)
There are other ways to use a sequence e.g. set the ID via a trigger, or if you're using an ORM you can tell it to use the sequence for generating IDs for this table. But right now this is probably the simplest approach.
Oracle sequence values are guaranteed to be unique, and there are various additional options e.g. for caching sets of sequence values, specifying the incremental step, re-cycle point etc.