Here is an example of how to create a sequence which you can use for creating sequential numbers, typically for ID’s.
CREATE SEQUENCE person_seq
INCREMENT BY 1
START WITH 1
This is how you would then use the sequence.
INSERT INTO persons (id, name) VALUES (person_seq.NEXTVAL, 'Dylan')
INSERT INTO persons (id, name) VALUES (person_seq.NEXTVAL, 'Isaac')
If you want to select the next ID you would do it like this.
SELECT person_seq.NEXTVAL FROM Dual