You mean giving a record a unique identifier? Use a sequence (in this example for <col_id>):
create sequence <seq_name> increment by 1 start with 1;
when you insert:
insert into <table_name> (<col1>,...,<col_id>)
values (val1,...,<seq_name>.nextval))
If you need to reference the just by nextval created value use <seq_name>.currval.
Stefan